I would run SPI rather then I2C (You can still use the chip addressing stuff on those parts, but see the errata for details of a silicon bug), just because I2C is in general general pain in the arse, costs an extra IO pin but it is IMHO worth it.
For relay drivers I would also see if something that would reduce your parts count was available, maybe a MIC5841 or similar, gets you the relay drive transistors and catch diodes in a single package that can just be daisy chained from board to board, note that this would require a series connection of the data line so it may or may not be a win over addressable parts with an IDC bus connection.
Over 8 inches at low data rate I2C would in fact probably be fine, with stiff pullups, I just hate having to write the state machine.
Regards, Dan.