Cool livestock for a 40g would include cleaner shrimp, blood shrimp, squat lobster, tuxedo urchin, pistol shrimp, watchman goby firefish, zebra or scissortail dartfish, neon goby, barnacle blenny colony, stigmatura blenny, midas blenny, grammistes blenny, purple tilefish, sunburst anthias, yellow coris wrasse.
These are just some possibilities, you can't get all of them, pick what you like and add slowly.