Usually if we had mutinies we would be a standalone warband without any affiliation to the race / chapter one is serving, and that would be bad. I understand you are looking for an way to add this but, i just cant figure out a good way of having it in there without destroying fluff as well as making it viable.
people create f2p account and shoot random friendly player? trowh grenade in friendly player? don't care to be banned because they are f2p? it's so orky!!!!
My first reaction would be YES, have all the traitors! Unfortunately the more I think about it the harder it seems to make work. Unless the developers allow individual strike forces to defect to whatever faction they want then it can't really work. But if that's the case, the game is just going to make rainbow colored armies for all sides and the factions will just be meaningless.
It could still work, It depends if we will have inter faction conflict (Which is actually viable for all of them).
I do believe that Friendly fire is a necessity. This is going to end up as a team game, if your team is shit your going to feel it.
It's more for balance than anything else. Can you imagine how strong Plasma Cannon spam would be without it?
Orks and Eldars dont mix with other factions. SM-CSM can interchange tho only one way mostly ^^; Orks always turn on one another due to several reasons Eldars... no, not in the seven hells they would turn on themselves ^^; Same rule for SM. CSM... propably... ? Also planting a spy would be quite difficult. Could work only for SM-CSM (again...) since how the heck would you want to plant an Ork into an Eldar place? It would look ridiculusly out of place ^^; If it were only humans it could work. But 4x totally different faction. An SM cant be among Chaos without risking of turning ^^; A CSM cant be among SM... his tained nature would blow his cover immediately (remember, most of the CSM warriors were in the Eye of Terror for 10k years x) Mutations, twisting and other nice stuff is just too apparent XD). That... and they would NEVER wear a loyalist chapter's armor ^^; Tho the feeling is mutual + the only way an SM would go rogue is if he turns to chaos. These elements are usually hunted down and killed on sight in lore ^^; SM chapters dont like renegades X) It would be nice to have a leader system tho, similar as there was one in RF Online and other games