Not quite. Arc at front, Amp driving ceiling speakers for rears (need care over impedance). But non-Sonos subwoofer cannot be incorporated into that setup. 

You could use another Amp instead of the Arc to incorporate a powered subwoofer. But that would need passive speakers, of course.

What about Arc, Amp, PolkSub, and only the BackR and BackL?

You can only use the Arc at the front, and you cannot use Polk sub with that. When a Sonos Amp is used for rear speakers the sub output is non-functional.

Thanks for your help. What if I scrapped the sub? Could the rear Polks function as rear SS with the Arc being the center channel?

I think you will have to start working with Sonos as it is designed to be used.

The Arc does front left, centre and right. The only usable subwoofer with the Arc is the Sonos Sub. For the rears you can use an Amp plus passive speakers or a pair of Sonos powered speakers. That's the design.