Depending on your betta, they may or may not tolerate shrimp.
I've had moderate success with my halfmoon, who shares a 5g with a small colony of red neocaridina (cherries); however it isn't a large colony, and I suspect that he snacks on shrimplets on the fly despite me not having witnessed it directly.
Varieties such as the hmpk's can be a little more directly aggressive, but it really comes down to the fish' individual personality.
I'd recommend shrimp from the genus neocaridina, this includes (red, blue, snowball) etc, they only grow to an inch max, usually smaller