You can use any tank you want on any one of those. THats the great thing about phantoms. They are practically infinitely configurable. The 12g changers are actually removable, so you can buy one with a 12g changer, and take it off to shoot with a tank. If you want a red dot sight (won't help at all, just sight down your barrel), you don't need any mods. It bolts right onto the sight rail. The extra left/right/center feeds are so you can change your setup. Not the best way to do that though. The best way to go is to buy a stock class (no feed neck, just a stock class tube) and then buy an old style spyder feedneck to bolt onto it if you want to use a hopper. Look
HERE to see what I'm talking about.