I would either junk the betta idea, or plan your entire tank around them. Bettas can be finicky, need lots of cover and plants. Plus, you have to be careful introducing them to one anotehr, as they establish hierarchies.
When you put them with other fish, make sure they are fast enough to get away when the betta girls get grumpy (actually, for this danios WOULD work well.) Slow moving fish may get harrassed by the betta girls.
IF you do get female bettas, do lots n lots of reading on 'sorority' tanks, and then either buy them from a breeder who already has them living together, or go to the LFS, and pick out ones that are, again, living together. Watch the tank, and make sure they are all peaceful and happy (and healthy). Then buy the whole lot. (My lfs gets in on avg 3-6 at a time.)
If you do it any other way, you may end up running back to the store a few times, or setting up another tank, just for that one female betta that is getting harrassed, or harrassing the others. ultimatebettas.com is a good reference on them, lots of articles.
I definitely recommend a small species of pleco as a bottom feeder. I love my two. And cories, or even certain loaches, are GREAT fun to watch. Especially if you go with nice peaceful upper level fish- like tetras.
I'm eager to here what you pick!