Alot of decor and hiding places help break up aggression but even then, I am not sure why mine get along so well, especially considering one of the females turned out to be a very young, mislabled male. Yes, he is still in there and yes I know males/females aren't supposed to live together but thay have lived together for about a year now and never have I seen any real aggression from any of them. He flares and sometimes chases that's about it.
Even with 4 or more females, there is no guarantee they will get along. Some are just too aggressive. I had one girl like that.