Firstly I suggest you get that watchman a pistol shrimp to watch. They can be a tad noisy, snapping away and all, but they won't hurt anything and it's awesome to see the tunnels they make all over the place.
Hmm... If you like silver and black, you could try a baggai cardinalfish.
Royal grammas are a bit territorial, but as long as they have their own cave, they're fine.
As long as none of your fish are less than an inch long, a rooster waspfish would work just fine. They get to around 3" long and are fairly peaceful, though they flare at anyone who's roughly shaped like them and will eat very small fish or shrimp. One warning on rooster wasps; their spines are venomous. You won't die if one tags you, but you won't be happy. As long as you keep an eye on where it is while sticking your arm in the tank, you're just fine. Rooster waspies come in a red/white/silver/rust mix and a dark red.
You could try a barnacle blenny.
Hmm... Maybe a Hector's goby? They hover instead of acting like regular gobies, and they nibble on hair algae, which is a bonus. They're also adorable, peaceful, and small. Your watchman might flare at it, but you should be fine in a 40g.
Chalk bass? They're supposed to be peaceful and are definitely colorful, though I've never kept one myself.
I do suggest you google the things I'm suggesting, find out more about them.
