xi32, There is nothing wrong with using a variable scope with higher magnification. Just keep in mind that some of the lower end variables will show different points of impact at different mag settings (test it yourself to see if it is worth worrying about in your gun), and that at higher mags, you will have lower light transmission for dusk/dawn shots, and may lose some field of view, and your exit pupil will get smaller limiting your eye relief.

It'll work fine, as long as the scope is rugged enough to deal with 12g slug recoil.

When using a variable for hunting, I suggest either keeping it at a single setting that can work across your entire range of expected shooting distances, or (if your POI does not vary) keep the mag turned down for close shots. You will have time to turn it back up for a longer shot. You just don't want to have a buck step out from behind a tree at 20 yards and all you can see in the scope is brown fur because it was set at 9x.

Good luck.