With Bettas and Neon tetres, it comes down to two things:
Different Bettas have different personalities. Depending on his aggresion, they could attack the Neons- Or leave them be. Unfortunately, there's no way to tell this until your Betta is actually housed with the fish. When you get the proper tank, I reccomend trying the Neons out, and if you see the Betta attacking them, return the Neons to the store.
Neon Tetras are schooling fish, so you need atleast 5-6 of them together. A 10 gallon with lots of hiding places is the minimum size for a Betta with these. Good luck!