I would start by getting the exact tank dimensions.
I have a young pair of angels in a 20 high (24x12x16) and they will need to move out soon. They are probably 1.5x silver dollar body size.
Realistically, no more than 2 angels in a 20g high...a 29g would be better, and long term a 30 long or even 4' tank would be ideal for 1 pair (2 pair might work in a 4' tank, but often not.
There's a decent change the angels will kill their tankmates if they decide to breed, or each other if they are all males.
With a single angelfish, you could possibly do a pair of rams (small ones like Germans/golds/electric blues) and a small school of tetras. But the small school of tetras may get eaten by the angelfish in time. And larger tetras won't have enough room.