I think it would work as well on the sole as it does on just heels and toes! If you were doing a rib or lacy stitch on the top of the foot, I think I'd stay with the smaller needle for everything, just to make life easier. I'd suggest using the smaller needle(s) on just the bottom and keeping the bigger needle(s) for the top if you were doing stockinette or a not-too-stretchy stitch on the top of the foot.
On second thought, I'd try the smaller all the way around on stockinette, too, maybe working one or two less decreases in the gusset to compensate.
You could be on to something here!