Adding alot of plants will aid in keeping the neons feeling safer. If you get a nice Betta they will be fine, if you get an aggressive one they you will most likely have to move the Betta to it's own tank. We have three Betta's in tank, two of them are nice guys and can handle other fish, one is a terror and lives alone with a couple of cory cats. That is one thing, Betta's almost always accept Cory cats in their tanks. Since the cory's are not brightly colored and swim on the substrate the Betta's, even mean ones tend to ignore them. But mind you on rare occassions a Betta comes along that can't be kept with anything. Try it and if it works great, if not as already stated, have a back up plan.