Do they have to be in a pair? No, they can be in singles.
What can they be kept with? Here are some general guidelines, they may not apply to your fish especially during breeding:
1) Fin nippers are a big no.
2) Fish that grow to big and could swallow mr angel whole are a big no.
3) fish around 1 inch or smaller could be preyed upon by adult angelfish.
In my opinion you will see the most interesting behaviours when you have a group, six or more. Obviously they need an appropriately large tank.
Typical tankmates are
- tetras selected from the chunkier species, such as cardinals and rummynoses, which won't be tasty morsels for the angels
- small cichlids like rams, which will occupy the lower part of the tank
- corys which will occupy the surface of the substrate
- hatchetfish, if you have a covered tank, as they are notorious jumpers.
I've got a breeding pair in with 12 Cory Sterbai, 10 Cherry Barbs, 1 Bristlenose and a Sailfin/Common Pleco. 260L tank. Probably wouldn't go anything smaller than Cherry Barbs but I have been keen to try Cardinals. I started with 6 and let them naturally pair then rehomed the other 4.
A 3ft tank would be adequate as a breeding tank in my opinion provided it was at least 16 inches high. I definitely would not go lower than that though.
50 Gallon would be better though if your going the community tank route.