Neons are finnippers and not a good choice with a Betta. The Betta tail is like cavior to fin nippers. Cory Cats will work great and they will help keep the bottom clean.
Think of the tank in horizontal layers of: Bottom layer, Middle layer, and top layer.
Let the top belong to the Betta as they are top feeders. Many will eat from the bottom as well but Bettas like to mostly hang at the top.
Give the bottom to things like Cory Cats that say on the bottom except when they dart to the surface for air.
The middle layer could be for any schooling mid layer fish like a Platty though I have had some problems with platties and Bettas.
Just don't overstock the tank. Think 1.5 inch of fish for each gallon of water your tank holds.
have fun and good luck!