Brake callipers with different sized rotors

I'm upgrading my brakes from cheap Shimano mechanical disks to Juicys. I can get a good deal on 8" Juicy 7's at the LBS (much cheaper than the 6" ones). I don't really want 8" rotors, and definately don't need them. My hubs are centerlock. Can I keep the 6" Shimano rotors, and use the 8" Juicy 7 callipers on them?
Thanks
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Parktools :?:SheldonBrown
Just to clarify: all the callipers are the same, and the only thing hat differs betwen the 8" and 6" set is the rotors?
Thanks
For example, HFX9s are deeper pads than Hope M4s so you can use Hayes discs with the Hopes but the other way round is not recommended.
Personally I can't comment of the interchangebility of Shimano and Avid as I've never had either.
Parktools :?:SheldonBrown