A 12x48 is rated for 1.7 Cu. Ft. of media with a 50% freeboard (50% of the media height, not 50% of the tank). Assuming you fill the bottom dome (Domes are not considered in tank media quantities).
GAC is usually backwashed at 10 GPM per Sq. Ft. 6" x 6" x3.14 / 144 x 10 = 7.85 GPM, you should use a 7-8 GPM flow control on the Clack valve.
Garnet is an excellent underbedding for GAC, just be sure it completely covers the bottom screen. Properly, it should fill the dome, but I doubt 16 pounds will do that. The Dome on a 12" tank is .13 Cu. ft. it will barely cover the bottom screen depending on what type of screen you have. #8 Garnet is approximately 150# per Cu. Ft.
Backwash frequency depends on the water use, but considering the high level of iron, I would assume weekly or every other week should be sufficient.
As to the Clack or 7000, both are excellent, and both should last 5-15 years between rebuilds. If you are using a water softener valve for the clack, be sure to plug the brine, and remove the brine piston from inside the valve. Not critical, but proper.
How are you injecting your chemical?
I am doing the 5800SXT rebuild next, I also am redoing the 7000 with some cut-away views.