Unfortunately it's difficult to give advice on what you should add to your aquarium because there is a potential problem with it's current inhabitants. Silver Dollars actually get to be about 8 inches, and therefore will need a bigger tank in the future.
For a 20 gallon tank you should look for fish like smaller (approx 2-2 1/2 in max) tetras, danios, rasboras, ottos, and corydoras. For "centerpiece" fish look for a pair of dwarf cichlids like Bolivan Rams, Apistos.... or go for a smaller species of Gourami like Honey, Dwarf, or Sparkling. Unfortunately with such a small tank you'll prob be limited to only two or three species of fish to ensure that you have room for a proper number of them. Many schooling species prefer to be in groups of 6 or more.