Yay! Sororitys are lots of fun, and seems like you have the basics down xD You've read the little thread that DQ wrote about them, right?
I'd say 4-5 would be a good number for a 10
Lets see.....as for behavior, its REALLY difficult to judge character in those little cups, or even if the girls you get are in a Sorority already.....when I got Mira she was at the bottom of the chain and beat to a pulp, however now shes top dog and the most social. You won't know who will work and who won't until you get it going from my experience.
When I look for a new girly, I look for health and color that strikes my fancy mostly.....activity level I find doesn't tell me much when they're in those little cups, none of them are ever really active....its not until I bring them home and get them into someplace warm that I see their true nature. But I do look for the basics, are here fins damaged? Is she swimming properly when I tap the cup to get her attention? Is there anything out of place on her body or fins? You know, things like that xD
Best of luck with your Sorority!