I thik it depends heavily on your betta and your tank set up.
I wouldn't trust my bettas with a shrimp personally.
You could put like a pair of Endlers (as long as they're both male) in a 2G with shrimp. They're really small. I know the tank recommendation is 10G for them but really really small. My thinking is that same as my guppy, they have /all/ the water space and in a larger tank there's no way they would have a whole 2 to 3G to themselves.