Normally I would suggest otocinclus to go with a betta, but to thrive you would want maybe 5 or more and that's pushing it for a 5g aquarium.
Maybe a mystery or nerite snail, or an adult size Amano shrimp.
I would really feed both separate foods rather than the same. But I've been a fan of bug bites for a betta, and with specific food for invertebrates, think something like homemade snello for snails , and I have always done well with sinking shrimp pellets for shrimp like from hikari... But algae and vegetables work pretty well a lot of the time too