If you got neons, you wouldn't be able to get very many, maybe 3 or 4 and I don't think they would do good in that small of a school. I wouldn't get any less than a half dozen neons, and I think with the mollies it would be too over crowded. I'm not sure how big ottos get. But with the two mollies and the betta, that is about 6" of fish right there so you really would only be able to get one cory and that is not enough, you should get at least three. I suggest keeping the betta with three of the mollies and that should be good. Anything more than that I think will be over crowding.