Well, I am pretty sure the 'random monsters' thing is just something that Hartman said in an interview to appease the moral guardians because there are at least three instances I can think of off the top of my head where ghosts are explicitly stated in the show to be deceased humans. Plus, Hartman wanted it to be lighthearted so regardless of its audience it would've been the way it was, I think....and it was rated TVY7, where a lot of popular cartoons today are TVPG and thus can get away with a lot more. Who knows! BUT HEY, if you're craving more of it then it's a good thing I'm here to FILL THAT EMPTY, GNAWING VOID IN YOUR LIFE

OH DUDE, Satanists too?! Man you are really gonna have 'em all, aren't you? I'd suggest some more but I don't know much about any real life religions, I can only further propose silly ones. ....Any Pastafarians in there, maybe?