I wouldn't put too much more in the tank without the fear of overcrowding.
If you decide to, a good choice is add a pistol shrimp to pair with your watchman.
Other choices would be the following, especially if you feed frozen;
- Scooter blenny (mandarins are not a good choice due to food requirements)
- chromis
- royal gramma
- another firefish (red or scissortail)
- cardinal (bangai, or pajama)
Ken - BWA