I have heard of people putting as many as ten girls in a ten gallon, but that was with a higher rated filter and many live plants. I have six girls in mine currently and two more in quarantine. This will be my final number, although I do plan on adding a snail at some point. BUT I am running an Aqueon Quietflow20, so I have twice the filtration I would typically need for a ten gallon. I also have live plants and keep up with my weekly 50% water changes.
Sororities are great fun, but remember to keep it to all girls! The male would probably be killed, and even if he's not, he'd promote a LOT more aggression. Also, make sure you have a backup plan in case any of your girls aren't suited to sorority life. Some fish just can't handle it.
Back after a hiatus.
Pongo - HMPK Koi Male
Onyx - DSH Cat