I have never heard the problem with the primers of certain case types that you mentioned before. My understanding is that the problem with the floating firing pin comes about from improper or insufficient cleaning, and often happens when the cosmoline and/or carbon from storage and/or shooting has not been properly flushed from the firing pin channel in the bolt.
As for ammo, It’s widely believed, though somewhat anecdotal that steel-cased ammunition is harder on your extractor. The steel in cases is harder than brass, but it should still be a little softer than the hardened steel of your extractor. This isn’t to say that i recommend steel cased cheap stuff like Wolf all the time. I think that “spammo can” surplus ammunition is a great, cheap alternative to Wolf, and should serve you just as well. Being older ammo, it is highly corrosive, and therefore makes cleaning your SKS all the more important.