From what people on here have said they are pretty hardy. They mostly eat fish, so any tankmates are pretty likely to end up being eaten at some point. I'm not sure about tank size, but they get relatively large so I would assume much bigger than a 50g.
If you're talking about the Hawaiin or japanes dragon, enchelycore pardalis, it's super hardy, 50 gallons would be pushing it, I'd recomend at least 70 or you're risking it jumping out of the tank. They eat meaty, fresh food from your supermarket (mine likes shrimp, squid, softshell crab, as well as small boney fish like smelt and sardines; it all depends on what's on sale .
If you're talking about the Muerena species (brazilian, mexican, etc) the rules are similiar, just make sure you know what species you're getting...some sp get larger and require at least 100 gallons, and at least one species shows up occaisionally that requires much cooler temps and won't last above 70F for any period of time.