Both the bala and the silver dollars require much larger tanks, 55g for the silver dollars and 5ft 120+g for the bala sharks.
To answer the question of which cichlids are appropriate for your tank size, you are looking at cichlids along the lines of, dwarf species, bolivian rams, german blue rams, keyholes, apistogramma, curviceps, hondouran red points, convicts to name a few.