This is what bettas has in their tank:
15 gal
2 rummynose tetras
1 serpae tetra
1 x ray tetra
3 male guppys
a male platy
1 flying fox
1 peltobagrus ornatus
4 neons
2 glowlight tetras
You don't have room for any more fish. Particularly seeing as you still have that flying fox that needs a much bigger tank. A lot of your fish are also schooling fish that should be in larger groups - but don't add any as you don't have room for them. Actualy, without the flying fox, your stocking would be fine - but you still wouldn't have room for anything else.