Mineral tanks are "fiberglass" tanks. They are polyethylene inner shells with a fiberglass wrapping, then epoxy coated.
The 12x52 is a better choice for a 2 cu. ft. system, the 12x48 is slighly small for this much media. Both are commonly used but the 12x52 is preferred.
As to you plumbing size, code does not allow for a reduction in pipe size. Water velocity is the key issue here for pipe size considerations. Internal piping of a system is an engineered item specifically designed to accomodate a flow velocity exceeding 10 FPS, so internal manifolds will usually have an exception to the pipe size rule. Also, inspectors do not know whats inside of a system, they only know what they can see.
I see no reason not to simply get a 2 cu. ft. 7000 w/ the proper 1-1/4" connectors. You may have to have a plumber install it, but it will be done right and to code. (hopefully)
Stainless Falcon flex connectors are available in 1-1/4", you would only need to sweat on a couple of MIP adapters to the existing plumbing.
Since you are in Orange County California, you will have copper plumbing, Fleck has 1-1/4" sweat connectors available at a reasonable price pt#41243-01 or 1-1/4" plastic threaded connectors pt#40565-01. Anybody you order the system from should have these parts available. Dont forget, you need to order 2 pieces.