You've got a bunch of options!
Midas blenny is a great option, they like to swim up in the water column unlike most other blennies.
Fang blennies are also open water swimmers, ORA breeds a great variety of them.
Flasher wrasse or smaller fairy wrasse would be great - maybe not a lineatus, but certainly something like a solorensis would work.
If you find one eating in your LFS and you have a deep enough sandbed a leopard wrasse would be nice. I've had a small meleagris in my RSM 130d for the better part of two years now.
The small hogfish would do nicely. A candy or a peppermint would be your only realistic options. They can be mean, though.
The fish in my avatar is fantastic - Pink Streaked Wrasse. Similar size and behavior to a sixline with none of the attitude.
I've had good luck with Fridmani Dottybacks, they're much more mellow than their relatives.
I'd stay away from more gobies, the only one I would try in a tank that small with a yellow watchman would be a cleaner goby, either a sharknose or a neon.
Firefish get a bad rap as cheap beginner fish, but they're great little open water swimmers.
If decide against the royal gramma there are other nice albeit pricey basslets. Assessors, swissguard, swale's, blackcap if you can find a healthy one at the LFS.
