Females are fine. I have 5 together and nothings wrong with them. But before you put them in, make sure you introduce them first. They will fight at first so watch them okay? But if you have a net, you can poke them or push them away with it to make the nipping start. Soon they will get a long. In a 7 gallon tank, you can keep about, 3. 3 would be perfect! That's because they grow 2 inch's maybe a little more. Well Good luck!!!!!
hope this helped a bit
hope this helped a bit