Actually I've yet to hear from anyone who has crossed proven genetic lines of ghosts and found them incompatible. Maybe I'm just not getting answers from the right people but the few cases of incompatibility I have heard of so far seem to all involve at least one unproven import. My theory is that some imported "ghosts" are caused by something other than genetics. It's also probably true that not many crosses between different lines have been done due to fear of incompatibility so certainly I can't guarantee that crossing a het yellow ghost to a het orange ghost can produce ghosts but I'm not sure that anyone can guarantee it can't either.