Bicolor blenny will perch on the rocks. Cute fish. A fang blenny will swim out front. I like the canary/fang blenny personally. Look'm up as there are others too. Yellow, blue and yellow, etc. On the gobies, you can do a clown goby (yellow or green most available). Small and perches also. Or a shrimp goby - he'll hang out out the bottom near an opening to a cave. If your rock is on the bottom of the glass and not on the sand, he and a pistol shrimp together might be nice too.
"Life may not be the party we hoped for but while we are here we might as well dance!"