well, i can't say whether gnome and kde coexist happily but they do coexist
i have some serious stability issues with kde, but i installed gnome first so i odn't know whether they're inherent in kde or because of gnome. also, i'm running the current unstable of both kde and gnome so that might be the problem
the install gnome, install the bundle-gnome package. its a convenience package -- nothing actually in it, but it depends on all the files you'll need to run gnome