I woul spend the extra money and get a complete set of 207rr's or regular 207's the 204 imho is a shit tire, different profile than the 207 as well it will muck up your handling with a more round tire in the rear (204) and a more pointy one up front (207)
I do not mix and match. It is worth the exter to get a new set. I get mine from Tire Warehouse (I'll post the address when I get home) they have a package deal with 1 front and 2 rears (thats how much longer your front will last than the rear before the center flatens out. I run the reg 207ZR's on both my 90 and the 88 and love them.