My HK 416 takes all my mags (King Arms and MAG brands) fine. The PMags have no real functional advantage. If you like the look, go for it but you might have to dremel (I've never tried those mags).
JG internals are fine. No need to strip them out and completely replace them. If you want, open it up and grease and shim it properly and everything. You may also want to downgrade the spring (it probably shoots +400 FPS out of the box).