Diamond Gobies are the sand sifting ones. The ones that pickup mouth fulls of sand and sift through their gills. Dragon Gobies are "Dragon Net" gobies, ones like the Mandarin Gobie. Those eat copepods but don't sift sand. I had both, both at brine shimp and mysis shrimp.
The Diamond Gobie will cover rocks, but it's because they are making a little house/cave. Once mine did that, he stopped covering things with sand. He would just sift the sand. Good fish. Pretty much stay on the sand. Just make sure you get ones that will supplement their own diet with brine and stuff. Ask the LFS to feed them in front of you.