Some kind of blenny would have been my suggestion. The Cleaner wrasses are beautiful, and people have successfully kept them in the past, but FAR more people have starved them to death. The bummer is that they are collected frequently, and so their price isn't high. They are truly difficult to keep fed properly. I have seen some that ate frozen mysis AND enhanced brine shrimp that still wasted away over 6 months. Having worked at a LFS for over 10 years, I stopped ordering them into the store entirely, and I wish more people would do the same.
The neon goby would be a much better choice care-wise, however they are social, need buddies (3-5), and you would not likely see much of just one anyway. They are best kept in groups. They exude a secretion that tells carnivores that they don't taste good, and I believe I read somewhere it's mildly paralytic to other fish, but they would be so tiny, I would worry about my triggers swallowing one whole and potentially getting sick.
There are some nice gobies, and some nice blennys that would add some activity to your sandbed/rockwork, and neither option should compete for food with your other stuff (while both your naso tang and a blenny both eat algae, they generally get along just fine, and neither eats algae only). Good luck!