A myth in my humble opinion. I've had lot's of success in the past mixing brands/speakers. Just be sure combined ohms can be handled by the amp and speakers are rated for wattage they'll receive.
As a side note.... I finally found a Rumble 112 cab in mint condition! Pulled the trigger today - gonna be nice sitting under my 500c! Of course the obligatory pics will be posted upon receipt of the cab!![]()
I as well, am looking for a rumble 112 ext cab.

So I'd opt for the 210 or 112 (if you can find one)