Both angelfish and discus are shoaling fish. This means they live in groups, and have a need to be in a group. If you were able to find a true "pair," meaning a male/female that have paired up, that is different. But otherwise, a group of 5 or more of either should be acquired, and at the same time. Both species are cichlids, and once settled in the aquarium a hierarchy will form within the group; putting 2 or 3 in now and then adding another 1 or 2 later usually means trouble, as one of the original will "own" the tank. This aspect applies to many shoaling fish.
A 55g is suitable for a group of 5 of either, if you are careful and consistent with maintenance and there are live plants. The other fish are a problem, as has been mentioned in this thread. Aside from the size, there is also temperature. Discus need warmth, and not all tropical fish can manage with the higher temperatures. Angelfish are largely commercially raised and manage at normal tropical temperatures, although wild caught fish would also need the warmth. The companion fish must also be quiet and sedate, as both angels and discus are so inclined and can be stressed out by too much activity around them.
You can read more about these fish in our profiles; please do. There is a lot involved with either, it is quite different from a tank of guppies and tetra, believe me. Pterophyllum scalare takes you to the angelfish, Discus takes you to the basic discus. Both fish have more than one species.