A Fantail and a Black moor 20 gallon tank. really?

I have a baby fantail and a black moor, in an excessively small tank. I am planning on purchasing a 20 gallon for them, but was hoping that i could put other fish in with them. However everything I have read states that per goldfish I need 10 gallons. There is nothing i can put in with them ? It seems funny to put two tiny fish in a 20 gallon tank. I know they will grow to be big but that will take years and that they have tons of waste between the two. I really wanted to get some Cory (i think that's what they are called) for this tank but it seems that this is not going to happen. Any comments/suggestions are welcome!