Hybrids are referring to interbreeding between WWs and Campbells as they are subspecies (Phodopus sungorus sungorus and Phodopus sungorus campbelli), and can produce fertile offspring, Roborovskis are a different species completely although closer related than Chinese, Syrian etc they couldn't interbreed with WWs/Campbells as far as I'm aware.
I wouldn't put different species in the same enclosure, I wouldn't expect them to get on, even introducing members of the same species doesn't always work. I know that some zoos etc will keep multi-species enclosures but they are really thought through and specially designed so that each species needs are met. Actually there's an article here about cohabiting rodent species
http://homepage.ntlworld.com/jean.wrigh ... /cohab.htm by a hobbyist who keeps many rodents, his whole site is worth exploring, it's not really like pets though, more like a home zoological collection!