I'm with Fella on this. In a 36 gallon tank, you should think about having decent size schools of your tetras. One neon all be itself will be about as happy as one person stranded on a desert island. Consider scaling these up to groups of 6 or more for each species.
Brackish puffers would be wrong in this tank. You one option is a South American Puffer. These are relatively peaceful, schooling fish that get along well with fast-moving fish. There are no guarantees, but usually this species works well with things like tetras.
Cheers,
Neale