There are documented cases of SKS slam fire, so one needs to be careful and know ahead of time what to do if it ever goes full auto. The fatal slam fire accident I heard about had to do with comoline still being in the bolt and the person setting it down while it was still firing. A buildup of crud in the bolt might also cause this kind of thing as well as various mechanical problems. A slam fire event could be a long few seconds holding on (for dear life) until the magazine is empty. Which is a darn good reason not to have anything bigger than a 10 round mag.
Actually, some folks feel that "floating" firing pin semi-auto's should not be used with typical "soft" US boxer style primers. Sounds like what you may have read somewhere. Some feel that using such a firearm with certain CCI hard primers is OK.
Others, like me, load up 7.62x39 ammo using winchester brass, Remington 123 grain bullets and soft winchester large rifle primers.
I have two SKS's and got them for my sons when they were younger and wanted deer hunting rifles. The short stock fit them and gas cycling system combined with the 7.62x39 round kept the recoil down. I worked up accurate hunting rounds. But for hunting, I made sure that the magazine only had four rounds max in it. At the range, I just load one at a time (range rules for all rifles except 22's).
After using my SKS's I completely clean the rifle after each use and spray a good "crud cutter/lubricant" in the firing pin hole of the bolt each time and flush out anything that is there. I don't expect to encounter a slam fire condition, but they can happen.