Would it have been better or funnier if Adam had been a warlock too?

Why not? The more the merrier!

Adam WAS revealed as a warlock in the 8th season episode, "Adam: Warlock or Washout?". This was rebutted in the later series Tabitha, which not only made Adam mortal, but older than his sister. ohno:

Really? I didn't remember that. Didn't they say he was mortal for a pretty good while? Did he ever use his powers on the show?
I never saw the show Tabitha, but it sounds like they changed some things around.