JS, Pmags are good to go. The issue with aluminum "GI" style mags is that any damage to the feed lips can be hard to visually detect, but can cause malfunctions. Pmags are either broken or not broken--the polymer doesn't experience plastic deformation very much. They can fracture, but then you can see it. That said, they are hard to destroy. So, if you are not regularly running them over, stomping on them, or body slamming them against concrete, you should be good. Even if you do abuse them, they are $10-15 each.
7.62x39 is not optimized for the AR system. There are uppers for them, but some have issues. If you are willing to spend a little more, .300blackout gives performance pretty close to x39, runs well in short barrels or suppressed, and only requires a barrel change from the normal AR system (the case is based on 5.56).
The 15-22 had some issues a few years back, but I haven't kept up lately. You might want to do some research on that. Alternately, 9mm AR pistols and carbines are gaining popularity.