I've never been really sure exactly which REM monsters might be eligible for an exchange for a collab monster. Of course the exchange menu will show you which monsters can be used for a specific exchange, but I'm not sure which ones could potentially be used. Is it just REM monsters with a base/lowest form that's at least 6 stars? For example, R Odin and Bl Odin could be used in an exchange, but never Gr Odin (any evo)? Or is there more to it than that? I've saved some dupe REM pulls partly, if not only, because I wasn't sure if they might be useful as exchange fodder at some point, but I suspect that most of them don't actually qualify.