Chocolate betta, neons can do just fine in a 10 gallon... Ad it depends on the fish, most bettas can live with tank mates.
Did you cycle your tank first of all, if not it is gonna go downhill fast. Can you post a picture of the tank as well? The tank should be densely decorated so if the neons need to they can get away from the betta vice versa. I would also add like two more neons before adding the betta, it will help them feel more secure because they will have more numbers. In the wild they will have thousands swimming together, there is no way you can do that in most aquariums though lol.
Also wait at least 4 days if you cycled it, that way the bacteria will have caught up to the amount of waste being produced.