Had to google the tank quickly, have you blocked that vent towards the lower section of the tank that leads to the filtration section? I ran a little Fluval Spec 3 that had similar, once I blocked that off and replaced the sponge with a DIY media rack I didn't have a nitrates issue anymore.
I like the minimal rock look, leaves more room for coral growth, something like a Kenya tree, toadstool or finger leather would get some nice height off of that rock, could try a gorgonian as well I suppose.
I've had clowns, Tailspot blenny, aiptasia eating Filefish, Wrasses and Damsels. Personally I like to have fish that work in the tank so for the 16...
Pink Streak/ Pinstripe Wrasse
Aiptasia Eating Filefish
Tailspot Blenny
Would stick to leathers and softies with those fish, especially the Filefish.