From Ruger's Mini-30 manual
"USE ONLY FACTORY AMMUNITION LOADED
TO U.S. INDUSTRY STANDARDS"
I assume that to mean that you're not supposed to use the Russian stuff (Wolf, Brown Bear, etc.)
Honestly I think the worst that would happen is you'll wear your barrel out faster, but I don't know for sure.
Ruger only sells the 5 round mags that I'm aware of. One of the pitfalls of the Mini's is the lack of high capacity mags.
Have you taken the Yugo SKS's into consideration? They are a much better value IMO than the Ruger. They cost about $180-$250 and CAN shoot the Russian (read: CHEAP) 7.62x39, since that's what they are truly chambered for.
Jason