I wouldn't get Cory cats in anything less than a ten, and I wouldn't get tetras in less than a 15. Corys are cute, but they prefer to school with at least 3 others of their species, and they are extremely messy fish. The floor of my ten gallon is constantly carpeted with wafer shrimp pellet crumbs, no matter how many times I vacuum it. Tetras are too active to be in anything less than a 15. There has been success with tetras in a 10 gallon, so if you're feeling confident go for it, but they'd appreciate the extra room.
If you do end up getting a larger tank, go for the Cory cats. I've never had a problem with them, they are so cute and bettas pretty much leave them alone. I own 3 reticulated, and they are spastic little things. One of them has vastly outgrown the other two, but they still school well.
In your setup, I think you have enough room a few (2-4) cherry shrimp. Cherry are a little bigger than ghost shrimp, and more docile. They'll be big enough to handle any harrassment from your betta, and they won't kill other cherry shrimp in the tank.