Can you run 18\'s on the back and 17\'s on the front
yes. the main thing to look out for is keeping the rear wheel/ tyre circumferance as close to stock as possible as the speed reading on the speedo wont match ur actual speed.
usually if u run different size front and back then the tyre profile will need to compensate for the differance.
ie 17" front with 50 profile, 18" rear with 45 profile.
This rolling-radius calculator is very useful:
http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC%20calculator.htm