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.