I would prefer dimensions (i.e inches or cm) rather than the tank's gallonage. Does your tank have a heater and a cycled filter?
Species such as tetras, rasboras and danios should be kept in small groups, it is better to stick to larger groups of one or two species than mixing and matching. You could also have a small group of platies however be warned that these fish breed very easily and very quickly. A group of small catfish such as corydoras and even a small plec such as a small Panaque or Hypancistrus will be ideal for the lower reaches of the aquarium.
If you fancy a pair of cichlids then there are lots to choose from, consider a pair of the smaller species such as Keyholes, Rams or Kribensis.
