If that can help answer some of your questions:
Obviously not perfect as a guide, but it should be fairly accurate 
You can also stack about 9 on WoA with a few different pieces of gear, I know @RxJunkie did it with a Stormfang build
One of the main reasons for that is how much SUPPORT each DoT type has outside of Masteries.
Electrocute is by far the least supported, while bleed is the one that has the most
Here’s a little table I made for myself some time ago of sources of additional flat DoT damage: Weapon and jewel refer to augments
Affixes is for green gear, so since you might end up using legendary gear in those slots, that column is the least relevant (Except for poison which is the only one that can get flat on accessories)
| Damage | Devo | Weap. | Jewel. | Components. | Affixes |
|---|---|---|---|---|---|
| Electrocute | 0 | 0 | 30 | 0 | 18 1H, 32 2H |
| Frostburn | 32 | 22 | 30 | 0 | 18 1H, 32 2H |
| Burn | 30 | 22 | 30 | 5x2 | 18 1H 35 sh, 32 2H |
| Decay | 60 | 0 | 45 | 6 x 2 | nothing |
| Trauma | 88 | 25 | 42 | 0 | nothing |
| Poison | 28 | 25 | 48 | 13x2 | 24 1H, 35 sh, 42 2H. 15x4 Jewel. |
| Bleed | 91 | 22 | 42 | 10x2 | 33 1H, 35 sh, 59 2H |