I think a single cardinalfish's tall profile would compliment that tank well, and that he'd likely spend some time hovering above the rock. A percula or ocellaris clown would add color and motion near and around the rock, and a shrimp/goby pair would lend interest to your sandbed. A clown goby could add a pop of color right on the rock itself - but perhaps wait until you've got some coral for her to play in?