the 18" will fit so long as the tire is sized correctly, heed the advice you where given above and make sure you do not run two different size tires in the rear... the traction control will go berserk thinking you are going out of control...

the bigger worry is, make sure that the new wheels you have will work with the stock bolts on the crossfire... pop one in the new wheel and make sure you have 19mm of bolt exposed on the back side... definitely no more and not much less... could lead to plenty of damage...