If you have all males, they will go after any female that you add and she will get overwhelmed. I think that all guppies would be fine. They shouldn't nip each other. They are good with each other. If they do start nipping, separate them because one of mine got picked on and nipped, not by another guppy, but by a blue tetra, and died shortly after, so make sure to watch them. They should be fine, though.