Hi,
The impedance rating of the speaker and the setting of your Yamaha do not relate directly to the level settings for each channel.
Just go through the normal calibration for each channel. Use a Radio Shack SPL meter and a setup DVD and do a manual adjustment of each channel's level from your seating area.
Other than just leaving your Yamaha at the 8-ohm setting, there are no special adjustments required for using a group of speakers of different impedances. Each speaker or channel is driven by a separate amplifier in your receiver, so the impedances do not interact with each other.
Regards,
