Theoretically, you could run pretty much any size tire, but a few considerations.

1) Overall diameter should still be the same, so if you switch to a smaller rim, you need to get more sidewall on the tire (ideally)

2) The tires need to meet the minimum load requirements, as in able to handle so much weight per tire. This usually comes in the form of the width of the tire and, again, how much sidewall you have.

Worse comes to worse, run on your steelies.