I think it's the newer Optima choke system if it's a new gun. I think... The White Onyx 12 I just bought is Optima, and AFAIK the various current Beretta field guns differ primarily in external appearance, with different finishes, engraving, and wood grade, though the walnut on my White Onyx is surprisingly pretty and figured.
I also believe MCF means "multi-choke or fixed" but don't quote me on that.
There's no particular reason that a 26" won't work; it will just swing differently from a 28". Note that the weights on the US site are wrong. The different gauge guns are all different weights, as they should be. Check the international website for correct weights.
I know a guy who uses a 26" 20 Gauge SP V with an English grip for all sorts of things including SC and American Trap when he feels like it. He is smaller than average, and British. Hits targets quite well.
The SP V is a very attractive gun. I'm a sucker for CCH. I needed a gun I could use in the field, though, without trepidation.
Is there a particular reason you want a 12/26"?