Yes, you are extremely over-stocked. Yes, return the goldfish to your LFS and get a heater as the rest of the fish are tropical or sub-tropical. No, you can't add bettas - you can't add anything.
Actualy, if you want bettas (I'm assuming you want 4 females - the most you can keep in a 10 gallon - and you can't get more than a single male as males attack other bettas), return ALL your other fish to your LFS and get the 4 FEMALE bettas OR 1 MALE betta instead. Don't add anything else.