Corydoras need lots of room, a full 10gal. They are very active and like to be in groups so I would steer clear of them.
Snails from my experience have done badly with me. For some reason my betta has a thing against snails. They also create a lot of waste. But eat algae, if you have it.
Shrimps are pretty good, don't need too much room, but can easily become snacks for bettas. I would try to pick up ghosts, but from my understanding they arn't as 'hardy' as cherrys shrimps, but cherry shrimps are bright and are easily spotted by the betta.
I would suggest African Dwarf Frogs, they only need a bit of room. However, if you have hiding spots, they will hide, any you will barely see them. They also need food placed right infront of them because they are nearly blind. But, they are freaking adorable and kick up for air on the surface.