I had a previous one that did very well. It was a planted 20Long and all the bettas were introduced at the same time. Four of the females got along very, very well, without any problems. The fifth one they all terrorized until I removed her. Never had many problems, even with torn fins, once the underdog was removed.
My most recent one failed. I had them in a 29, introduced at the same time, and lost them one by one. I have one female left out of seven. However, I suspect this is actual due to an illness rather than fighting, especially since the ones I pulled were "alpha" in the tank. I know for sure 3 were ill before they passed. This was after I separated everyone from each other and other tankmates.
I agree with AshK's tips. You can take precautions but it's a bit dicey. Make sure you pick tankmates that won't pick on them.