A lot of fish can go with GTs but a 20 is too small for a single GT let alone anything else. With my GT I have 3 O's, various amounts of Jewels, 2 blue Eyes, 3 Synodontis Eupertus, 2 Plecos, 4 Silver Dollars, 6 Clown Loaches and a Chocolate Gourami.
They get along fine, but my tank is 200 gallons larger then yours, and I think its getting a bit crampt. If I could get rid of the jewels and my brother took back his loaches I would be fine.