For your smaller tank, how big is it? In general, ghost shrimp can live in a relatively small tank but I probably wouldn't keep them in less than 3 gal either.
Are your bettas male and female, or two females, or two males? Of these combinations, two females would be the "best" combination, but as has been mentioned bettas should really be kept by themselves since they are not very sociable with other fish.
Can you post pictures of your tanks?