TL;DR: Yes, a poison Cabalist is completely possible and viable. It may not do the highest single target poison damage, the crown for that still rests firmly with Witchhunters. In contrast, a poison Cabalist excels in AoE damage, and can clear a whole pack of mobs quickly, easily, and relatively safely with a combination of pet, AoE damage and CC/debuff abilities.
Now here comes the long part, so bear with me. I am currently playing a lvl 85 poison Cabalist, and I am absolutely destroying shit. I would be higher lvl, but I’ve leveled several other characters since Ashes launched, because I’m a crazy build whore 
But…since you don’t care about that, onward! So, here is a breakdown of my experience with biological warfare.
Necro Skills
Ravenous Earth: Completely destroys for most of Normal/Veteran. Although the radius isn’t huge, it’s very effective AoE because of how fast/how many times it hits. I am in love with this spell. The one major con to RE is that you have to get relatively close in order to cast it, so it’s best to cast it, pull mobs, then back up a bit. Alternatively, you can set your pet to aggressive, have him pull, then drop RE right on top of a pack. The upgrades to RE are optional, and a little bit lacklustre, tbh. Decay is terrible damage scaling for the point investment, I would recommend skipping it. Foul Eruption is decent for AoE, so it’s your choice if you want to invest points in it, or put those points somewhere else.
Blight Fiend: Ok I really love this guy. Especially the way he looks. ESPECIALLY the way he wobbles/runs! This is a fairly tanky pet, on par with the Shaman’s Briarthorn, but he can actually do some nice damage too. As far as the Blight Fiend’s upgrades go, Rotting Fumes is kinda meh, but Blight Burst is amazing. It’s like a mini version of the Tainted Eruption proc ability from the Abomination constellation, same graphic and effects, just lower damage. Max it.
Master of Death: This is a skill I have mixed feelings for, and I have to do some more experimentation with it. The damage scaling is nice, since it adds all pet damage, as well as poison/acid/vit, and increased OA. But, I feel like it doesn’t hold up well against Bonds of Bysmiel/Manipulation. You could theoretically take both, but it would only be possible to do so at a late stage of the game, probably lvl 80+, and your points might be too spread out. My gut feeling is that MoD is a skill that better serves a pure pet build, and since a poison Cabalist is more of a hybrid, Bysmiel is more bang for your buck.
Everything else in Necro is kind of meh for a poison build, but I will mention a few. Spectral Binding is worth putting a few points in for the health buff, but that’s it. Ill Omen can be very useful as a crowd control ability, and I did actually use this myself for a significant part of Veteran and a bit into Elite. Decide for yourself if you want to take it or not. Siphon Souls is a very powerful AoE vitality dot, and you could take it for a nice lifedrain effect when mobs rush you. But, Vitality is going to be a secondary damage stat in a poison build, so it won’t reach it’s full potential. Not worth the point investment, imo.
Occultist Skills
Dreeg’s Evil Eye: I only took Evil Eye when I was about mid-way through Elite, probably around lvl 65 or so. Ravenous Earth is enough to get you by quite nicely for basically all of Normal/Veteran, but once you get to Malmouth and when you move onto Elite, you’re going to want something that hits harder, which is where this bad boy comes in. I recommend taking Focused Gaze, and filling out the upgrade path as much as you can.
Curse of Frailty and Vulnerability: Essential. Max them.
Bloody Pox: I know, I know, this isn’t a poison skill. But, you’re going to want to take this. Because Pox does life percentage damage each tick, you can cast it into a group of mobs, watch it spread, then sit back and laugh as your poison AoE’s finish the job. Pox is especially necessary for some of the larger, tankier mobs and elites, since it’s damage is based off of the total life of the mob. Wasting is decent, but not really necessary I feel. Black Death, on the other hand, is godly, and it’s not because of the poison damage it deals (although that is nice) but because of its chance to confuse mobs. Highly recommended to take at least 5-6 points, if not more.
Blood of Dreeg/Aspect of the Guardian: Blood of Dreeg is incredibly useful, since in addition to being a secondary heal ability, it also applies to your pet(s). Use it often, use it well. Aspect of the Guardian is nice also, since it provides a small damage boost and basically makes you immune to poison/acid damage, which again applies to both you and your pet. However, it is not strictly necessary.
Bonds of Bysmiel/Manipulation: BoB is a huge boost to the survivability of your pet. I would consider this to be an essential talent. As far as Manipulation goes, if you can fit it into your build, it’s going to be one of the largest contributions to your pets damage, since in addition to increasing damage, it also increases the total speed of your pet, thus making it attack/use skills faster. Compared to Master of Death in the Necro tree, I feel like Manipulation is just a better investment of points.
Possession: By the time you fill out the other skills in your build, you should already have quite a lot of poison/acid/vitality damage scaling, but more never hurt anyone, right??? Well, except for the dissolved corpses of your enemies.
Constellations:
Rat: Gives you a total of +44% poison damage and 30% increased poison duration. You’ll probably have to take this as a stepping stone.
Bat: Vitality scaling. Taken as a stepping stone, but it does give you the Twin Fang’s proc, which is not completely terrible.
Akeron’s Scorpion: Gives you a total of +54% poison and 15% acid damage, and the celestial ability Scorpion Sting. Yes, it’s poison scaling, yes you get a proc ability. But is it really that good? No, not really. It’s up to you if you want to take this for a bit more poison scaling, or go for another high Eldritch Affinity constellation, like Raven. I found the proc to be lacklustre assigned to either me or my pet, and wanted to save skills for use with other constellations. Scorpion Sting was basically an unused, unassigned ability, so I skipped this constellation in favour of…
Manticore: Gives you +90% poison and acid damage and 30% increased poison duration, and gives you the Acid Spray proc ability, which does acid and poison damage, but more importantly, lowers enemy resistances. I recommend linking this to Bloody Pox for maximum coverage/chances to proc.
Raven: Not a great constellation for this build, gives a small bit of damage scaling for you and your pet, and a bit of OA. Taken more for the Eldritch Affinity than anything else.
Bysmiel’s Bonds: This is a very nice constellation. +30% to all damage and +5% cast speed for you, and +70% to all damage for your pet and +8% total speed, AND gives you proc ability to summon another pet. Solid choice.
Eye of the Guardian: An interesting constellation. Gives you +60% poison and +15% acid/chaos damage, and the Guardian’s Gaze proc. I assigned this skill to my Ravenous Earth, the reason being that since RE hits rapidly and multiple times in a very short time period, it maximizes the proc chance, thereby easily granting you a full stack of guardian eyeballs to thrash anything that comes too close to you. Taking this constellation, along with Manticore, will also provide you with the 4 points in Ascendant Affinity you need in order to take…
Affliction: A very nice constellation, which gives you a total of +40% poison damage, +50% acid damage, and +90% vitality damage, as well as +3% total OA. Additionally, you gain access to the Fetid Pool proc, which creates a ground effect dealing poison and vitality damage, and slowing enemies. I recommend taking only the right side stars, save the other two devotion points for something else.
Murmur: Now this may seem like an odd choice, but bear with me here. While Murmur does not give you any poison damage scaling, it does give you +90% acid damage, and gives you access to the Rumor proc ability, which in addition to dealing cold and acid damage and spreading like crazy through mobs, also lowers mob resistance to poison/acid, which stacks nicely with the resistance debuffs from Vulnerability and the Manticore’s Acid Spray. You’ll have to take some filler constellation for the necessary 6 points of Primordial Affinity in order to take this. I recommend either Gallows, Eel, or Sailor’s Guide.
Abomination: The crowning glory of the Poisonmancer. This constellation gives you a whopping +260% poison damage, +180% acid damage, and +80% vitality damage, and gives you the Tainted Eruption proc, which is absolutely devastating. Not only does it deal significant poison damage, but it’s a large 10 meter radius AoE that also confuses mobs for 1.8 seconds, providing additional crowd control. Now, you could assign this to a dot effect like Bloody Pox, which would maximize the proc chance because it would be a 15% chance per tick, per mob, or you could assign it to Ravenous Earth, which would again maximize the proc chance because of how fast/many times RE hits. But, the downside to those choices is that the AoE effect from Tainted Eruption is centred on you, and in higher difficulty levels, you aren’t going to want to get too close to mobs. So, I actually assigned Tainted Eruption to my Blight Fiend, and there are a few reasons for doing this. First off, assigning it to your pet will make the AoE come from your pet, not you, which I find much more useful. Since your Blight Fiend already has a mini version of this skill in the form of Blight Burst, you will see twice as many green explosions coming from the little guy! Next, the damage that Tainted Eruption deals is based on your damage scaling, which will be much higher than your pets, but the chance to proc the ability will scale off of your pets attack speed, which is buffed from Manipulation and Bysmiel’s Bonds. So, it’s the best of both worlds. Say hello to your undead biological warfare kamikaze bomber!
Behemoth: This constellation gets an honourable mention. I didn’t take it in my build, since I opted to take Murmur instead, but Behemoth is a valid choice, since it increases life for you and your pet, and gives your pet increased life regen.
Shepherd’s Crook: Another honourable mention. I didn’t take this in my build, but taking it would net you +13% total health for your pet, +15% resistances for your pet, and the Shepherd’s Call proc ability, granting +60% total damage and +10% crit damage to your pet. Not bad, but more useful for a full pet build.
Other Stuff of Note
Components: Vitriloic Gallstone, essential because of the poison buff that also applies to your pet. Mark of Dreeg will give you more poison scaling than another Gallstone, and gives you a usable skill in the form of a poison AoE. The skill is situationally useful is early-mid game, but doesn’t scale well after the mid-40’s to 50’s. Venom-tipped Ammo gives you a small damage boost to poison/acid, and another usable skill, but it’s not that great. Rotten Heart is a chest component that gives a poison/chaos damage boost, but you will probably want to save your chest slot for a resistance/life/armour component instead.
Augments and Factions: There is a ton to choose from here, so it’s really up to personal preference. Rovers, Homestead, Outcast, and Coven are probably the factions you’ll want to build rep with the most, and the ones that will sell you the most useful damage augments. For gear, Rovers and Homestead sell some decent mid-level poison/acid gear, Coven has some higher level stuff.
End-game Gear: DREEG’S!!! The Dreeg set is ungodly for a poison caster build. The Eye’s from the Black Gem of Dreeg spawn all the time, and actively seek out enemies. Additionally, every piece has a skill bonus for the Evil Eye line, which means you won’t have to invest as many points in yourself, if you are wanting to put those points somewhere else. The set bonuses are great, and the ultimate, 5-piece set bonus is just…wow. The only disadvantage to using the full Dreeg set is that you are going to have some trouble maxing our your resistances. Some other gear of note: Since the Dreeg set does not include leg armour, it leaves a bit of a gap. The best pants I have found to fill the gap are Nidalla’s Legwraps and Venomskin Legwraps. Gloves, Viperfang Grips seems to be the best choice, and has a very strong proc, so unless you find a good pair of green gloves, this will be your go-to. For rings, you have a few choices at different gear levels. For mid-60-mid 70’s, Ring of the Black Matriarch is decent, as is Empowered Ring of Eternal Rot and Empowered Ring of Vile Intent. However, because you have a poison pet, the crowning glory here is going to be the Rotmender’s Ring, which has a proc ability that heals you, while also providing damage scaling to both you and your pet. For amulet, you’re going to want to keep the Black Gem of Dreeg, but I will make special note of the Empowered Blightshard Amulet, which gives you the ability to summon a Blighted Riftscourge pet, and provides a poison damage boost to your pets. It’s tempting, I know, and I tried it out myself. But my experience with the Rift Scourge is that it just dies too easily, even though it’s a ranged pet. For medals, again you have several choices, but the one you’re going to want to try and get is the Rotdrinker Crest, which buffs both you and your pet.
Relics: Lvl 25, take Corruption, since there isn’t really a better option. At lvl 50, you can take Blight, which is a solid upgrade. The proc ability isn’t that great, but the damage scaling is quite good. At lvl 60, you can upgrade to Malediction, which in additional to giving you more damage scaling, will also give you a poison damage toggle buff which is very nice. At lvl 70, you can choose to upgrade to Dreeg’s Affliction. You will lose a bit of damage scaling if you do this, because of the loss of the Malediction buff. However, you will gain the castable skill Dreeg’s Afflicted Spines, which is a very powerful poison nova skill, like the Dreeg’s Infinite Gaze skill on steroids. Past that, there is a lvl 90 relic that has poison/acid damage scaling, but unfortunately, it’s not very ideal for a Cabalist, so you may want to just stick with the Dreeg relic.
