I had a very beautiful betta in my 69 litres tank. A week after I got him, I got a pair of moonlight gouramis (Trichogaster microlepis), and put them in the same tank. He flared at them and tried to keep his territory, but in the end that territory was some hidden places above my filter or between some plants. I brought the gouramis back to the shop after a week (and that's a really good thing, as these gouramis grow too big to go in my tank), but then it was too late. The betta died, maybe because he was exhausted. He had no chance against two big gouramis.
I don't think I'll try a betta in a community tank another time. I'd rather set up a small tank especially for the betta.