Only problems I've ever personally seen with bettas in communities have the bettas on the receiving end.
A friend of mine got this great looking large turquoise long finned male one afternoon to replace his recently deceased betta, put it in his ~40 litre with 3 black skirt tetras, couple guppies and platies, same community tank the old betta was in. The next afternoon, we couldn't see him. Found him hiding down behind the sponge filter with all of his fins chewed off. He's in a bowl by himself now.
I keep 2 bettas, a large red short finned male who's possibly the biggest betta I've seen in person, and a large blue female. They're in a 50 litre/13 gallon planted tank with 2 Bolivian rams, a blue ram, 2 honey gouramis, a pair of 4cm angel fish, 2 bristlenose and 16 kribensis fry who are around the 2.5cm/1" mark. My 'growout and miscelaneous' tank. Plant growth and 50% water change every second day keeps nitrate at about 10ppm.
The only fish in the tank I've seen with even slightly nipped fins is the big male betta.
