This rock IMO is on the heavy side. It has all the nooks and cranies that LR would have. I would use it as a base, not completly fill the tank with home made rock. LR has the bacteria and other stuff that will move over on to the HMR. All you would need is a few pieces of LR instead of #'s.
I think the idea is a good one to use the 10 gal. as main and the 55 as a sump. It would be stable, you can have a dsb in the 55, only a few corals would be needed to fill it up, only a couple of fish, less $ spent on lights and other "stuff".
It could be done very cheaply without being cheap. If you want out in a few months you would make most of your money back. Word of caution, once you get the bug, theirs no turnin' back.
Good luck.