I'm not sure about adding too much more, for a 20 gal the fish you have is a lot. Maybe when you get the 37 you can trade some of your old fish like the blood parrot and something else so then you can get the rams an Cory's
The Red-Tail shark shouldn't be kept with some of those fish, or in a 20 gal, because he'll eventually out grow it. Maine is right on the overstocking, good call.
__________________ 20 Gallon Rio Orinoco Biotope (High):
If you are absolutely desperate to add fish, go with two corys. Despite what people say, they actually are happy as long as they have a buddy. But everyone is right; your tank is way overstocked.
__________________
Fish seller and breeder
Hit me up at my online aquarium store!