You may find some ideas in the sub forum Betta fish compatibility
It depends on the size of the tank, and whether your tank is filtered. Anything under 10 gallons it is recommended not to have other fish in with the betta.
For 10gal + tanks, platies, cories, guppies, mollies are good with them.
Cories are good at scrounging for left over food, but you will still have to feed them, as well as still siphon out the substrate and clean the tank regularly. There is no magic cleaners other then just bottom feeders of food, unfortunately.