Just don't use the mix of 18 and 36Gb drive in the SAME volume. That is bad. I don't see why you can't have different disk shelves of same drives, ie. a shelf of 18s and another shelf of 36s, attached to an F720. We have this config on an F760.
NO, what you must not do is mix disk sizes in a *raid group*. Its fine to mix in a volume as long as they're in different raidgroups. I think the confusion arises as many peoples volumes don't span raidgroups. Another thing I would advise against is mixing sizes in a shelf.