I wouldn't recommend any fish friends in anything under 10g. But price is different depending on where you live and the availability of fish. Most fish require 5+ of the same type to be happy. I'm sure someone may come along that has a solution for you, but as for myself, I wouldn't add fish :)
Shrimp would be fine, but remember that some Bettas like to make snacks out of them. I have 6 hybrid shrimp in my 15g with 6 female Bettas. They're doing ok :) Shrimp like lots of cover, places to hide and do well in larger groups 6-10. But yeah, I'd say they could be added no problem. Small bioload.