12 inch slicks on a 10 inch rims?

They'll be fine. Nothing wrong with that at all. Also, there's NOTHING you can do about 2nd rib tread wear on a bias ply tire. Nothing. It's inherent in their design. Always has been. Course with slicks, there are no ribs, but still you'll see a difference between the first 4" from the outside edge. It's just the nature of the beast.

My new 295 60 d15 hoosier quicktimes were designed for an 8-10" rim , hoosier states it in their info sheet. I have them on 8" prostars, and with 26# of air (setting in the garage) they appear to have a full 10" contact. I can always lower the air pressure a biy to help it.