I agree with what other members have said. An angelfish if healthy and in sufficient space will grow easily to six inches in body length, and with a vertical fin span of 8+ inches. This requires space to develop properly.
In your 20g, a pair of rams if fine (I assume they have bonded and are not fighting each other) and you can have some "dither" fish, like a group of cory catfish perhaps, or a small group of small upper water fish. Ember Tetra would be good, as they are small and you could have a group of 8-12. The more "normal" tetra frequently get too large to work here. Pencilfish are also good fish for rams, though not all of the species work. The dwarf pencil, Nannostomus marginatus, is very good for this.
Byron.