tell us the size of your tank and we can help more.
I have a 5 gallon tank on my desk at work with a betta and 6 neons and tnhey are fine, very attractive. I get a lot of compliments. Also have some MTS snails, and I'm thinking I see either pond or ramshorns snails too. I'll have to thin those.
Yep, depends on the size of the tank. I have a betta in my 36g with small rainbowfish, platys, kuhliis, neons, glolites -- all sorts of fish, but none of except the rainbowfish are known to be "nippy" types, or might be nippy. The rainbows don't even pay attention to him. I also have a LOT of plants and hiding spots and lots of space for the betta to go and things to do.
He's far happier in this tank than he was in the 2.5g he was in. That's for dang sure.
Avoid fin nippers (most barbs) or fish with long, flowing fins (male fancy guppies) or other labyrinth fish (gouramis). A 5 gallon seems a bit small to add too many fish... a betta will love it, though!
I have had my betta with kuhli loaches, zebra loaches, rasbora espei, zebra danios, otos, snails, and cherry shrimp. I think it can be pretty individual on the betta, though.