Do you have female or male bettas?
I had 2 females, but one was a bully and had to seperate them. (To little space can cause this too, that being said from experience.) (I've heard some can get 4 females to get along in a 10g, others think that's not near enough room.)
Not sure how much space you would need, but when I had some neon tetras they got along fine with the betta.
Had some rosy red minnows in with each of the girls at one time or another, and they usually got along. (The bully did pick a little when there was just one minnow, but did less so when there were 3.)
The neons were only ever in with the gentler female, the only time I saw her pay them much attention was when I first put them in. She went right up to them and 'sniffed' them, it looked so funny LOL!