Welcome to my wonderful world of multiplayer building. It is a joyous time, and you can achieve great heights by going down this path. Also see this comment, where we teamed up to try and see how high we could super-boost a pet’s damage.
You’re absolutely right in thinking that you can create synergy between multiple players, between shared defensive buffs, and synergistic debuffs on enemies, like stacking RR.
Your instincts to combine Oath/Inquis/Soldier/Occultist for phys damage and survivability are good.
It all depends on what sorts of playstyle you want with them, though. Do you want melee? If so - dual wield, sword ‘n’ board, 2 handed? What about ranged builds? Or one of each? Pets, casters? What about retaliation synergies? What do you and your teammates enjoy playing? The possibilities are virtually endless. 
It also sort of depends on your goal - Do you want to be able to clear things quickly, or survive things well? Do you care about being able to kill Callagadra or Crate? Do you want to be doing SR 150+ ? Or do you want to be able to do SR 75 very quickly? Or do you just want to play builds that ‘feel’ fun, and are not too concerned with certain in-game milestones?
All of these considerations can guide your efforts.
Alternatively, make no consideration, and just start trying stuff out to work out what you like organically. 