It totally depends on the type of fish you're looking to add - do you want schooling fish, bottom feeders, lone fish? If you want a school it'll of course depend on the size of the fish and how many need to be in the school. You have to look at the adult sizes of any fish you want to put in your tank, and then add that to the adult sizes of the fish you already have. Since you have room, I would really suggest you first work on enlarging the schools of the fish you already have. Your cory would love to have more than just one friend, they're schooling fish and much more active with more of the same species. Also, tetras are schooling fish that really need to be in a larger school to be happy - 6-8 minimum.
