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

The only cichlids you could really get for a 20 would be one of the dwarf varieties. The cichlids wouldn't go well with any of these other species due to both temperament and water requirements. Puffers don't usually do well with any other fish due to aggressiveness. Some species of loaches and barbs would fit your tank and go together fine. I would recommend a book such as The 101 Best Tropical Fishes by Kathleen Wood (Petsmart sells it). It has information on a lot of cichlids as well as many other neat fish.