Yes, you can put all the FSG BGLs into the same Scenery folder and add that folder to Scenery Library. The default method is just the easiest for most. If you want to tinker around turning it on and off, the procedure is a little more complicated, and can be as complicated as you want it to be, into regions, or whatever. As far as in-game performance, it really doesn't matter where you install it. Initial loading may or may not take longer, I don't know. Mostly depends on how many installed elevation points have to be loaded in order to create the scenery surrounding your aircraft's location
Yes, every FSG terrain mesh file begins with "FSGDem"