I don't know about 5 Betta's. I personally have a 5 gallon tank with two Betta's, a Black Skirt Tetra, and an Albino Cory. One Betta is a Crown Tail female, and the other is a baby of which I can only guess what it well end up becoming. I introduced the female after the Tetra and the Cory. She was all up on them for a second each. She just left them alone once she figured out who was who. The baby Betta went in last, no surprise when the female went up and poked it. I caught the female today, doing what I can only guess as, trying to take care of the baby Betta. I guess you could call it nesting, but I need to read up more on it.
Of all the reading I've done, females are supposed to be the most docile of the two sexes. Reading the other post, I think it might also depend on the fish itself and the breed. Crown tails seem to be ok as a female.
READ UP ON FISH THAT FIN NIP
. Betta's can fall victim of fin nipping. You want to avoid most Tetra, I think I got lucky with mine, but I've been checking the tank every day now for the past week to make sure every one is getting along. There is a whole list of fish that like to nip at fins. This page gives a good idea http://www.bestfish.com/nippers.html
Other websites and litterateur I've read suggest avoiding colorful, aggressive, and highly active fish. Stick with browns, off white, grays, and blacks. Also try to stay with fish that like to hang on the bottom of the tank. If they are less likely to bump into a Betta, then there is less of a chance for there to be a problem.
In my case, I have plenty of plants and a bunch of foggy glass half bottles and cups for shelter and hiding places. Create a well rounded environment, not just a desert of rocks, and I would imagine creating a community of fish with the betta's shouldn't be too hard.