No you can't put anymore fish in the 10G with the goldfish.
Fish don't just grow to the size of the tank their in. Whats happens is waste produces Ammonia- Ammonia turns to Nitrites, Nitrites turn to NirtAtes. NitrAtes in large proportions are growth inhibitors(thats why daily water changes on fry). Your fishes growth will slow down but not stop and eventually it will be cramped in such a small tank.
So buy that beauty a bigger tank

also when you buy it a bigger tank and some friends make sure they won't fit in it's mouth because it just might eat it's new found friends

Carol