Wheels

I have two 18" x 10" rims for the rear of my car I was going to by 18" x 9" wheels for the front. I was wondering if there were and other advantages to getting 2 different sizes besides handeling or anything else you want to share.

Depending on the rim weight can be a factor. All factory Mustang rims are HEAVY!!!! Also a larger rim will allow for larger brakes depending upon the offset. Factory brakes are one common weak point in all Mustangs. Even the 00R was under braked for anything other than light track use. Going to a lager size rim and tire you will notice the tendency for the car to "track". Also going to a larger rim you will notice some rubbing issues when the wheel is a full lock to the left or right. The 03/04 Cobra eliminated this by the use of a pin restricting the amount the wheels can be turned but this in turn increased the turning radius.

I have lowering springs that need to be installed still (Ford Racing C springs)
so I'm probably going to have even more clearance porblems right?

Not unless go with a rim that does not have the correct backspacing. The clearance issues I was talking about only happen when the stearing is fully turned to the right or left. Now I am not to sure how wide a rim you can go with in the back before you run in to issue with clearance but agian that would also be dependent upon backspacing.
http://www.miracerros.com/mustang/t_suspension3.htm#Wheels