Pmags are Polymer and the mechanics of an airsoft mag are a lot different than real steel ones. Real steel mags are pretty simple compared to airsoft ones...
I've always like metal airsoft mags not because the insides are metal or anything but the shell around the whole thing is metal and they won't come apart like I've seen a few MAG brand mags do (that were unintentionally overloaded).
And as far as the single/double-stack mag thing I really don't know enough about regular AEG mags to tell you what kind of mag you could get for your particular gun that would be single or double-stacked. I honestly didn't think they made single stack mags apart from really low-cap 60 round Marui mags or something (which are metal btw. great mags). All I really know about these days is the PTW stuff.
Oh yeah and I don't think a little silicon, graphite whatever on your hop-up is too bad. After you fire a couple rounds that stuff usually seems to come off anyways and your hop is back to normal so don't get too worried about it.