Do I understand from your question that you want to put 18'' tires on the rear?

Note that the rear wheels are larger than the front, so if you put 18'' rear wheel, would have to raise the profile of the tire so as not to alter the development.

Put hard tires I think not successful, the Crossfire is a car that easily insinuates the rear, would be dangerous to put hard tires.

All you can do serious question, although I imagine that the car's stability would be affected.