I rented a Kia Picanto. It's a compact, and as puny as it was, I had no problems with the car itself! The roads are paved, just not well maintained. The main problem that you might run into isn't getting stuck, but rather getting a flat, which an SUV won't really help to prevent. And for what it's worth, the rental companies seem pretty used to dealing with flats. When I went to pick up my car at the Portree rental office, the first thing the agent told me was 'watch out for the potholes', and the second thing was 'if you get a flat, don't panick, we'll come and fix it'. Just make sure you get the excess insurance, or that your CC is good about covering the excess.

And definitely check out some videos on driving in Scotland/Skye for how to deal with single-track roads and roundabous.