Ah. I thought about that, but then I just figured that if you pushed for the "bad" demon, he'd just take up that same slot that WF is in now.
IMO, a couple of the slots were wasted. Kang the mad wasn't really a follower, since you couldn't use him in combat, and thus seemed better suited as just someone you'd meet at the "base" whenever you wanted to fly somewhere. And the Genie or whatever who sold you stuff was also a waste. I didn't really like Henpecked Hau either. He added comedy, but in terms of actual usefullness he was boring.
Off topic slightly, but something ironic. I had no clue who any of my NPCs were going to be, so I didn't know their names when I created my character. I named my guy Kang. So it was a bit of a shock to see someone else with the same name join me as a follower. :D