20 gallon aquariums are a fun size. They are small enough to fit on desktops and entertainment centers, but just large enough to house some pretty interesting species, including shelldwellers, rock-dwelling tanganyikans, kribs, apistos, a few of the more peaceful mbuna, convicts, and some of the dwarf south american cichlids.
Keep in mind that 20 gallons is still quite small. Many people have a tendency to pack small aquariums with lots of fish despite their reduced size. This can be problematic because the bio-load may be too much for the small volume of water and degree of filtration. In this situation things can go wrong very quickly. While certainly many more options exist for a 20 gallon Long, we strongly recommend that the beginner not venture beyond these suggested fishes (and numbers). This is particularly true for the Malawi cichlids. And note that a "pair" refers to a group of male and female, not two unsexed individuals.
A reliable heater is an important investment for smaller aquariums. The water temperature can fluctuate much more easily with small volumes. Stability is important for happy and healthy cichlids.
All of these fishes will feel most comfortable if provided with a fine, sandy substrate. The color does not matter too much, but a natural color is preferred. Shells are necessary for the shell dwelling lamprogines. Rocky crevices or ceramic caves are recommended for the julies, leleupi, and apistos. All of these species are egg-layers with the exception of the mbuna from Lake Malawi.
Filtration may be very simple. A small hang-on-tank power filter is what we recommend, something such as the Emperor 280, AquaClear 200, Penguin 170, or Eheim Liberty 150 or 200. A bio-wheel is a plus. Submersible filters are fine, but keep in mind that they take up valuable space where space is already on short supply.
Only Apistos and those species from West Africa and Lake Tanganyika are "plant friendly." The others are serious diggers and/or plant grazers.