1) Cracking teeth cannot be avoided! 2) Paraloid is easily reversible. If you want to seal immediately, you can soak the skull in acetone if grease does emerge. Personally, I'd wait a week or two though.Also, you can paraloid the teeth and leave the rest of the skull unsealed if you like. Just paint it on with a brush.
I would assume, on a fish mount, that it will leave a glossy surface. It sinks into the surface of bone and other porous materials and doesn't really show.