I would leave the spare on if it fits fine under there. If you ever get a flat on the front, just put the spare on the back and move your back tire to the front. The back can fit a wheel as small as a 16" if you still have the stock sized brakes.
It will at least get your and your precious cargo home. But I would not want to keep driving for a long time with two different sized tires on the rear, as that might mess up the differential. But I would at least drive on it to get home or to a tire shop.