You can still tinker with the mechbox if you want, but unless you know what you're doing it's really not suggested.
The Systema mags are heavy because they're real steel mags.
For the most part Ranger Plates shouldn't require any modification of the mag itself (other than bending the arms a little on the bottom of the mag) but pulling the floor plate off the mags is a real pain in the ass. You'll also have to knock the pins out and pull the internals out.