The safest advice to give is to tell you not to add anything else. That said, in a tank that size you need an animal that doesn't need much space. Dwarf angels need more than 20g. Adding another clown could be really tough at this point, because the one you have has already established the whole tank as it's territory. There is a very likely chance the new clown would get destroyed by the one you have now (I know, it doesn't seem aggressive at all, believe me they ARE aggressive towards other clowns). I selected a cardinal fish. They don't need much space, and are docile. Another choice is a firefish, look into them, the only concern is it may not get along with your other goby. After those choices, I'd stick with some other kind of invert, maybe a serpent starfish.