If you get the neons AND bettas you can't keep anything else. There's no room.
If you go for the neons, you could get 6 neons and 3 cories or 1 or 2 dwarf gouramies and 10 neons. You could also have guppies or platies with neons. 3 platies/guppies and 9 neons would be a possibility or you could have 2 male guppies, 3 cories and 5 neons.
If you went for dwarf cories and neons you could keep 4 dwarf cories, 7 neons and a dwarf gourami
or 2 male guppies/platies, 7 neons and 4 dwarf cories.
You could also add some more shoaling fish like harlequin rasboras and have a tank with 7 neons and 7 harlequin rasboras.
The harlequin rasboras (or neons) could also be replaced by black phantom tetras, cardinal tetras, golden dwarf barbs and rummynose tetras - all of which are beautiful shoaling fish and most are also hardier than the neons would be.
If you went for only female bettas, I think you could keep up to 7 in there.