Royal grammas really enjoy having a hidey hole if you can make that happen, but honestly it'll probably make residence in 1 of the holes of your rock if it's small. If you are looking into perching fish, a geometric pygmy perchlet has similar behavior to a hawkfish but its small size and being an anthia vs a true hawk makes it safer with shrimp--especially if you go with a larger tiger pistol vs a randalls (though they move a lot of sand for a tank your size). Lawnmower blennies also can sometimes struggle without a lot of grazing room in small tanks, but it's a dice roll and you may get one of the ones that gorges themselves on frozen meaty food.Thinking of the following fish for this tank (written in the order I want to add them)
1) Yellow Watchman Goby (with Pistol shrimp)
2) Royal Grama
3) Longnose Hawkfish (may remove as i hear they can hunt shrimp and that would suck for the YWG)
4) Lawnmower Blenny
5) Pair of Clownfish
?(6)? (Maybe add a Sixline wrasse last. However not sure if this is to much for the tank. Also heard they are sometimes D#@ks lol.
Don't get a sixline especially in such a small tank. Theyre demons even with lots of room. If you're looking for a fish that will be up in the water column maybe consider a dartfish or firefish of some kind.