3 otos (they do best in groups of 3+) in that tank would ensure enough space for all your fishes. I would not add the neons, they will probably either get killed or nip the bettas fins.
I read about somebody that had fish that was constantly nipping their betta's fins, they said that AS SOON AS THE FINS ARE ALL GONE, he thought they would stop nipping because there wouldn't be anything left
Bettas usually like to live alone, but in some cases bettas and fish can be mixed. The reason otos are good for betta tanks is because they are generally not seen. If you do decide to add neons (which I wouldn't suggest), make sure you have another tank for them to go in if a problem erupts. Good luck, Brianna