Don't get an angelfish, please. They eat the tetras, which is sad. In my tank I have neons, black skirted tetras, some glowfish and a few cleaning fish. They are happy as clams. Try some other tetras. They are very happy like that.
Don't get anymore, it is already too crowded. The cories should be in groups of 4 or five, anyway. If you upgrade to a larger tank, then a nice centerpiece fish is a male dwarf gourami: http://www.thekrib.com/Fish/gourami.jpg
EDIT: Do NOT get a goldfish, and I wouldn't get a betta either because they can be aggressive to neons sometimes.
I think that you should get a beta. They have long absolutely beautiful tails and they should only be aggressive toward other betas. You can find cheap ones in the fish section at walmart. At a pet store with fish you can find the regular veil tails and sometimes the crown tails. If you go to a breeder and don't mind spending a bit more you can get a half moon. These are absolutely beautiful.