It's fine to put rock in a qt (in fact, some people qt rock to make sure no pests hitch hiked in). The only thing is, if your fish in qt does have something, you'll want to remove the rock before you treat the fish and depending on what your fish has you probably wouldn't want to move the rock to a display tank... so you can get into a bad position that way. In a perfect world, we would all have 10 qt tanks that we could qt everything seperately in isolated systems. Obviously, most people can't do that.