Mixing 8 and 6 Ohm Speakers with a Yamaha RX-V659 / HTR-5960

I'm a Yamaha RX-V659 owner for all of 24 hours. For the moment, I've got it hooked up "old school", to just my two "actual old school" Genesis 33 speakers. I see Yamaha makes a big deal about specifying 6 or 8 ohm speakers, and the 33's are indeed 6 ohm, so that's what I've selected. I suspect most all of my others speakers (some still to be acquired) are 8 ohm. There's no mention in the manual about mixing 6 and 8 ohm speakers - is that because it's a no-no?

Is there a means to convert 6 ohm speakers to 8?

Appreciate any input...