The biggest issue I see would be rating lines: if everyone uses the ones from the Mundane, they'll all have +2 Charm. Probably the easiest solution would be to say that they should just allocate +2, +1, +1, 0, -1 to the ratings as they prefer.
If I was going to do this, I'd also tell everyone to pick one move from the Mundane playbook (and possibly include some other non-supernatural moves) to start with, and let them deal with the first adventure only slightly better prepared than a normal person. Then, after the first mystery is solved, have everyone pick a playbook to represent what kind of monster hunter they'll be and go through choosing moves and stuff as normal.
Make sure you tell everyone to make their character concept someone who will become a monster hunter when they find out monsters are real.