Just to throw a potential spanner in the works.
Watchman gobys are great fish, full of character and cute. As long as you can see them that is.
Since we bought ours 4 or 5 months ago we rarely see him. He found himself a home at the back of the live rock and we only ever see a fin or tail while he's chasing food. Once in a blue moon he treats us to an appearance at the front of the tank while he sits on a rock for 5 mins or so then disappears till the next blue moon.
We have since bought a pink scooter blenny and he is just exactly what we expected the Watchman to be but he is visible 95% of the time. He is quite active over the rocks though so I don't know about suitability for your size tank, maybe others would care to comment. I suppose the good thing about the Watchman is he doesn't move much!
Cheers