I recently started a sorority, it has 4 females, and one male (sorry to the people who I told I wouldn't do that), the tanks got plenty of hiding places, and is heavily planted. There was some fighting going on originally; mainly the girls establishing dominance. The male is definitely the alpha of the tank, and he got in a fight with one of the girls over an decoration, which I took out temporarily. I put the decoration back and there hasn't been any fighting since. I just started the sorority so anything could happen still, but so far the tank is doing great, I havn't had to quarantine anybody yet :)
I guess what I'm trying to say is that keeping males and females is possible, but I also really think it depends on the temperament of the fish itself, the male I had never flared up or showed any signs of aggression before I put the other fish in there. I have a Betta in a separate tank and he is always flaring up and he even kills the poor snails that live in his tank. I'd never put him in a tank with other Bettas.