Found the first problem with my layout. The pool cleaner booster pump T is in line before the SCG, before the valve for pool/spa. My daughter cranked up the spa for the first time at 11:00 the other night then came in a little later to tell me the spa is running out of water. A significant amount of heated "spa water" is going out the Polaris, into the pool. I could replumb it all, to raise the height of the SCG and give myself a little room to move the booster to after the spa/pool valve, but instead I'll add a small valve in line with the booster flex hose.
Other than that, it's working great. I'm loving NOT hauling chlorine and being able to leave town for a few days without fear of a green pool. It's running at 55% and keeping things clean.