IMHO any of the fairy wrasse species would be a good fit, and they are all very spectacularly coloured. A pair of Hawaiian flames would do well in there (I kept a pair in my 65g). Just be wary that they jump (all fairy wrasses) when startled, so it's really a question of what's keeping them in the tank when (not if) they jump. A cover or full canopy is a must.
In case it helps with any inspiration .. I also have in my 65g a C. ulietesis or double-saddle butterfly, a golden dwarf moray, a Siganus doliatus or penciled rabbitfish (3", small), a flame hawk, and a diamond goby.
Other suggestions ... flame angel or potter's angel .. blue or yellow assessor .. black cap basslet ... orchid dottyback .. blennies (ooh, maybe a tailspot blenny or midas blenny? lawnmower blenny maybe?... or if you can find one, an ember blenny .. hooooooly mooolleee those are nice!) ... dartfish ... gobies .. cardinalfish ...
