I know this is old post, but to help others on the 20" wheels you can use SCC Super Z6 SZ435 or the Z-575 with no problems. There is plenty of clearance to fit them and the Z-575 don't touch the wheel if you're worried about scratching the wheels. I've driven a few hundred miles in Big Bear with them this season, and they work great with no problems.
Before anyone wants to bash me for using them, if you don't know they are required to even go up the mountain when anysnow or ice is present. Put them on the front by the way.
Skinny tires on snow is an interesting comment as well, and for the most part incorrect. Snow driving boils down to tires, not what car you drive although AWD can help but only as much as the tires allow it to. The stock pirellis weren't built for snow, but an all around tire, so they will suffer after the tread wears down a bit.