SLG Glorious Leader Dan Vado went through the whole attraction, finding any and all names/characters/pictures/statues/etc. that could be used. He made a big list of them for all the creators. I can't remember how many there were, but it was less than a hundred.
At one point, I wanted to do a series about the "Museum of the Weird" that was part of the original design, which would have added more characters. I'm sure we could have easily kept things going for years.
I hate to break the news but issue #8 will be the last one, so it looks like we'll never know how many licks it takes to get to the center of a Tootsie Pop (if "licks" is "stories" and "the center of a Tootsie Pop" is "the 999th ghost.")
Wow, that's a pretty obtuse metaphor.
Anyway, we we're pretty much allowed to come up with whatever we wanted, Disney just had to approve it. (They only wanted me to make one change on my first story; that the streching room was being stretched supernaturally, not by mechanical means). It was an easy fix (and actually led to the idea behind my "Pickwick Capers" story, where we see the ghosts stretching the room).
Of course, it was easy to pick existing ghosts instead of make up new ones, which is why I called all three of the hitchhiking ghosts as quick as possible.