Communal, but with a few exceptions:

Newborns probably need to be kept separate so they don't get mistaken for food.

Those species that tend toward cannibalism, obviously, like the Wandering Garter. If you do get cage mates, make sure they are similar sizes, and keep them well-fed.

Keep males and females apart, as has been suggested above, but especially if you keep multiple species. Cross-breeding is highly discouraged, and so only keep males and females together if they are the same species.