I have a very similar tank. I've heard my tank described as an 18 gallon show tank. It is 10"x20"x20", it looks like 2 10 gallon tanks stacked on eachother.
I've done well making it a planted tank, with some tall plants and some long driftwood pieces placed vertically. Right now its got 1 female electric blue jewel cichlid, 1 golden zebra loach (Botia histrionica), 1 zebra loach (Botia striata), 3 serpae tetras (Hyphessobrycon eques), 3 tiger barbs (Puntius tetrazona), & 1 headlight-taillight (Hemigrammus ocellifer). There are "rules of thumb" for keeping schooling fish, but I've had this mix for a while (the headlight-taillight & serpae tetras being the veterans) and they all thrive.
There is a lot you can do with your tank. mollies and other live-bearers are another option. But I would suggest planting the tank. Its a smaller tank, so supplements for the plants will be very little and a bottle of flourish will last most of a year. You can get by with dosing with excel instead of needing CO2 injection, and the height allows for taller species of plants to grow. As far as fawna is concerned, you have many options for small schooling species, you ca do dwarf crayfish, various small shrimp species, fiddler crabs (if you give access to air via tall driftwood or other means.
