I am not sure how much 1" will make a difference but, you will likely lose a bit of SPL.
If you will be adding a 3rd woofer down the road, be sure to use the SW12 16FR's, the 8 ohm version will not work with 3 woofers.
With duals or triples, either the A370's or the smaller HX amps will work, Danny/Brian at Rythmik can guide on which will work best for you. If you think you may go larger (quads/6 stacks at some point), then the HX800's or the A370 that can be run in master/slave are what you'd want.
Yes, as SJ mentioned, we do offer a modular design, Rich (HAL) is gluing up some additional modules to expand his duals to triples now.
Post 253 here is where the details on the modules start, some additional pics on folloowing pages
https://www.audiocircle.com/index.php?topic=139480.240jay