You can rotate wheels, you just need a spacer on the rear wheels. If you bought a set of 18x9's with a 30mm offset, you could use them all the way around with an 8 to 10mm spacer in the back (because then the effective offset would be 22-20mm). That would take longer wheel studs. Or, if you found a set of 18x9's with a 25mm offset, you could probably get by with a 3 to 5mm spacer in the back, which likely would be fine without longer wheel studs.