You don't actually have to buy them all at the same time, just most of them to get it started. I would start with 5-6, good numbers.
And the good thing about the tank is that if it's got enough plants, you can do a Silent Cycle instead of the normal Fish-In Cycle or Fish-less Cycle. I implore you to look up the Nitrogen Cycle if you don't know what it's about
The best thing you can do for a sorority is have a heavily planted tanks, females can be just aggressive as males so there will be fighting. Sometimes you'll have to step in and put the bully in a time out (see through breeder box will work) so that they can see each other but not touch. Generally they should stay like that for a week or two weeks until they calm down.
After everyone's in their place and the pecking order is all set, you won't have to do much for them (except the tank cleanings and stuff but I'm talking behavioral stuff) Here's a great link on sororities: http://www.bettafish.com/showthread.php?t=123796
And here's another, it's a little more "scary" but explains fine: http://bettysplendens.com/articles/p...articleid=1097
As far as QT'ing them, I have about .5 gallon plasticware containers that I use, changing the water either everyday or every other depending on size. And I float them in my big tank, it's a 33. I do this for 2 weeks (generally QT for 2 weeks, if you can do more though that would be better just to make sure there are no diseases being transmmitted. It's a lot easier to treat one fish rather than 20) so that the girls have full sight of each other and when I put the new girls in, I follow the first link's advice and feed the older girls up front while I release the new girl in back.
I haven't had a problem yet doing that, however right now I've got three new sisters going in and what seems to be the oldest one is pretty intent on killing the baby sister. So I foresee putting her in time out for a while once she enters the sorority. She'll be last so that the other girl will have time to adjust to the sorority because so far none of my old girls really seem to care much about her lol, they're all interested in the feisty girl XD
And you could easily fit 15-20 girls in there if that's all you had with a small school of cory's at the bottom. Right now in my 33 I've got 13 girls with 6 Dainty Cory cats and 1 Bristlenose pleco and I could still get more girls