you have a coule of problems, one the damsel, you need to get something that will not be bullied too much by him, so firefish, or any gobies are pretty much not an option to me
and the size of your tank, you can not house a very big fish with what you hae, so you may even want this to be yourlast fish, may be one more, depending on what you choose
my choices(if it were me), would be a dwarf angel(slmost any, and flames are cool as well as cherubs), an algae or lawnmower blenny or a scooter blennythe blennies are pretty mcuh fearless, and fairly hearty, so i would personally opt for one of them, and if you want more color, my other choice would be a clownfish(any but maroons) like a perc. or tomato
HTH