depends on the tank size! smaller tanks, under about 10 gallons, you should stick with snails and inverts- and like someone said, the nerite snail is a great algae eater, is small, and no matter how hard it tries to reproduce, it's eggs don't hatch in fresh water. Other snails can work too, but be careful, some poop a LOT. in the shrimp department, i think your biggest risk is that they can become snacks.
when you get to like 10 gallons you can have other small schools of fish with your betta. if you want a clean up crew, cories would be a really good option, but they need schools so certain species would be inappropriate for a small ten gallon. pygmy cories can do well in a ten gallon.