As long as you haven't got a spool or locker in the rear axle, you'd be ok putting a different height tire on the rear. You could run a different height tire on the front too, as long as the front doesn't have a locker/spool, or if it does, as long as you don't run it in 4wd or with the hubs locked in.
The wheel size won't really make any difference in any aspect, it is all about tire height.