Well, 44 gallons with a watchman goby... I personally don't consider a reef (except nano systems) complete without a clown. Excepting perhaps the largest species (maroon, clarkii) you could get a pair. Cardinals are docile and offer a change from "typical" fish shapes. I'm find of royal grammas, too. Just be sure not to add more than one or two at a time to avoid overwhelming the biofilter. Also, consider a pistol shrimp. The symbiosis between the shrimp and goby is fascinating.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef