That's a great question....and I feel like you're probably going to have totally different responses.
From my personal expierence, I bought two female bettas together and had them in a 5 gal....switched them to a 10 gal and tried to add a third female Betta. They were very aggressive with her. So to answer your question if adding another betta could be a problem, yes other similar species could cause a problem. Even the dopes at the pet store will tell you that.
BUT, I have heard from owners that have males together, multiple females, males and females, bettas with other similar species. I think it depends on the amount of space, coverage, tank conditions, and what your fish's attitude, if you will, is like.
I hope that help in anyway. I'm having the same problems as you and I just find ten more questions to answer my original question. Good luck!