What fish go together??? Never owned a big tank?

20 gallons isn't actually that big. Cichlids are very aggressive so I wouldn't put anything else with them and they generally need bigger tanks. I have a red-blood parrot all by himself right now because he can't get along with anyone else. Ask the pet stores of if you go to petsmart they have their classifications on the tags, just be sure to to mix the types of fish.