Well I don't think mad science is out there either. Heck you could probably make the argument of Bible Black being in the same H Universe as Discipline which was pretty crazy in its own ways. Anyway you could make the Shadow World essentially the Hell the demons came from in the series. And we only saw one demon in the entire series. It might be possible to expand on that to include tentacle demons.
*Pondering some more*
Anyway it's just my opinion that there doesn't need to be two separate games. I would just suggest to players to adjust sheets so that it can make more sense.
Well, it basically comes down to compromising the setting for the sake of the players or not, and the gm's already going to have enough trouble running this(It's an unfamiliar system, and an unfamiliar style of game to him), so it only makes sense to keep the feel of a relatively mundane if somewhat perverted school with mysterious occult connections. Doing anything else would be incredibly selfish of us.
But since godfang is a nice guy who doesn't have the heart to tell us no, I suggested we split up into two groups. One for people who want to play as non human freaks or get played with by the non human freaks, and one for those who want to play as humans in a world where the occult is widely thought to be a hoax and demons are the only monsters we really have to worry about.
I can think of at least 3 demons from bible black, just from the original school, so yes, they were kind of common. One formed a contract with the nurse, one really weak demon possessed the main character, and one was summoned in the flesh by the original witchcraft club.