Well, for any community with a betta it needs to be planted. I'll hazard a guess that your betta may have been stressed with the neons, and he couldn't hide as easily. I have neons in with one of my males, and they get along. another option is dividing it is half, and putting another male of the other side, but you would have to be careful in case a male got through the divider.
3.5 gallon-Splendens Stelleri, the School Betta 5 gallon- Bluejay 20 gallon long--5 platties and Cleopatra. 2.5 gallon- Rose 55 gallon- 4 gerbils