PG is Propylene Glycol, VG is Vegetable Glycerin... base fluids. PG gives more throat hit & lets the flavor come out stronger, VG produces more vapor & is thicker, but tends to mute the flavor a bit. Try both. Yes, you can reload any cartridge, including the "pre-loaded" ones that come with the kit. I would suggest getting a bunch of small bottles in different flavors to try... include some
tobacco, since you'll still crave 'that' taste when you start, but make sure to get a variety of others. I gave away all my 'baccy flavors 2 weeks ago, I just don't like them anymore, but at first I did. Right now I'm on a mint binge, lol... that and chocolate-y ones.
Try a lot of different stuff, and then you'll know what to order in larger quantities when you make your second order
Oh, and I'd suggest a total of 30ml when you start. Even a heavy smoker should make it a week and a half with 30ml... plenty of time to re-order.