The thing with Tiffany is she's too much like Granny. By the time she's old enough to be in the main series (after wintersmith) I think Granny will be dead. Tiffany will be to young to fill Grannys role within the Coven, so it wouldn't work.

Or, if Granny is alive, Tiffany is to much like her for them to work that well in a main book. Unless, it's in the teacher/pupil role like Men in Hats.