The older wizard models (not on the GW site anymore) tend to be more "generic". They are painted in basic colors, not specific to a school.
Also, using the current Empire Wizard boxset, you could make a wizard with a globe, for example. It doesn't have to be a celestial wizard, paint the globe as a crystal ball or something. All wizards have those, right?
My point was mostly, if you like the model then use it! However, you don't have to have a wizard with a fur coat if you want to use beasts magic. Conversly, if you love that model with a fur coat, there is nothing to say that he cannot be a fire wizard either.