My male betta shares a tank with 4 zebra danios, 5 neon tetra and a pitbull pleco.....and he's absaloubtly fine, at first i had to take 2 extra danios back to lfs for picking on the fighter, though given time not sure if they would have stopped, as the others were nosy, maybe a little nippy at first but now leave him well alone....the tetra don't go near him either and the pitbull plec is scared of everything....lol.
I was advised time and again against this set up.......just make sure you have a backup plan, like being able to section off part of your tank for the betta, or another tank for him, just incase either he is too aggressive to the other fish, or that they fin nip him!
I have heard cherry barbs make good betta friends....as apparently to white cloud mountain minnows....tetra are renowned fin nippers...but like i said...it all depends on fish personalities as mine are fine!, when i got my betta he was in lfs in a tank of platies and was fine with them, half the reason i got him, cos i knew he could stand up to more bustling fish
just do your research, remember no long finned fish that the betta may mistake for other bettas....like mollies....apparently gourami are a bad idea as both bettas and gouramis are antabanoids and are territorial, because i was planning on getting a gourami to but decided i didn't want to add a possible problem, people in the betta forum also advise against angelfish as they can be aggressive too.....however swordtails can also make good tank mates for bettas, though i have no experience of that.
good luck!!!