it sounds like a good thing to try, although in the long term I think it would just end up being 1 particular comp dominating every time. Then again I've never enjoyed how stale the barb/druid/nec/sin meta was but alot of other people seemed to love it so maybe theres nothing wrong with that :P
I say go for it, you could even do like 3 teams of 2 ffa if only 6 people join or just a 2v2 if only 4 join.