Both the Zombie and the Ghost gimmick, do not change existing game mechanics, but add new level components (with their separate rules). In particular, turning these gimmicks on for a level that does not use zombies/ghosts has no effect at all. So there is no sense in requiring specifically turning on the Zombie/Ghost gimmick each time, when one wants to use them. Adding zombies/ghosts is not straightforward in any case - no need to make it even more complicated.
Suggestion: Remove the Zombie/Ghost gimmick from the gimmick list and allow zombies and ghosts in each level.