Female Platys
I have 4 female platys, 1 male guppy ( the other 2 died) and a pleco all in a 7 gallon tank, and the females get along fine. One of my Red platys can get a bit "bullyish" at dinner time, but they all seem to get along great, and they are all different sizes. My guppy gets a bit tired of chasing my platys though, I think I'm going to have to get him a friend.