I introduced a betta into my community tank over a fortnight ago, and kept a vigil for the first day in case anything bad happened.
Basically, the gouramis (liste below) had a bit of frisk of the betta with their feelers, followed him around for a bit and then left him alone. My golden chased him for a day or two on and off, but only because the betta insisted on hovering in the path the gourami likes to hurtle along in front of the glass.
Now they all seem happy together; occasionally the dwarf or golden gourami will nip at him, but they do that with everyone (and each other) and it's never a full on attack/vendetta, more like 'I'm grumpy, keep out of my way'.
I seem to have a peaceful betta, as he never attacks anybody, just muddles along chasing his reflection and doing lengths.
I had a back-up bowl for the betta, just in case he didn't take, but no problems so far - all is happy!