What do you mean by larger monitors together? Are you referring to many of the same species, or more than one species in the one pit?
Although I have had no experience with this (have a monitor pit ready but not currently in use) I would recommend NOT mixing different species. There would be species that would occur together naturally, probably the Lace Monitor and Heath Monitor but I wouldn't want to risk injury, or possible cross-breeding. On that subject, do different monitors cross-breed if given the oppourtunity?
Simon Archibald