Arc & Amp with Existing Rear, Sub Speaker Setup

That would be fine. The Amp with third party speakers is a standard surround option with all Sonos HT products.

The Amp can power rear surround speakers with the Arc as fronts, however the subwoofer output on the Amp is disabled when used as surrounds.  

So, then I would have to purchase a Sub as well and could only use my rear ceilings?

Or, am I better off with just two Amps, one to power my current front non-Sonos speakers/sub and one for the rear. With that configuration I am unable to get anything more than Surround, correct? No Dolby anything regardless of the tv that I go with?

Dolby Digital is surround, and you would get standard Dolby Digital 5.1 (no Atmos) with that setup. It would be 5.1 through 4.1 with a phantom center. 

To enjoy your new TV investment with eARC I’d purchase the Sonos Arc, Sub and Amp to power your in-ceiling speakers as surrounds.  However, I’d wait until the Arc is officially shipped B4 purchasing the Sub and Amp in order to take full advantage of the Sonos 45 day return policy; if purchasing direct.

To clarify my earlier response, I took the 'non-Sonos' to apply only to the surrounds, and the Sub to be a Sonos Sub (because capitalised and no 'woofer'). @jgatie  is of course correct that a non-Sonos subwoofer cannot be used with an Amp powering Sonos surround speakers. 

Is there anyway of toggling the amp between surround speakers for arc and then ( using same speakers to another virus room for playing music - im preferring my Hugh end ceiling speakers for when I play music only. I can move the sub easier enough from room to room but it gets me to reset the Sonos Alp each time try to move it to a new room not to mentioning having to tune the surround every time I add it back to the arc as surrounds...alternately can u mute the arc speaker itself and have only music through surrounds …

There’s no easy switch within Sonos for devices from surround to separate or stereo operation. For this a new set up is required. 

Understood doesn’t make sense though - one hopes it’s fixed in future - simply need to be able to save surround settings and move Sonos amp to another room.  Sonos is better than this

With exceptionof the aptly named Move, Sonos speakers are not meant to be moved.

For Sonos multiroom audio means a speaker in every room.

You can use a third party sub (or other non-Sonos surround speakers) with the arc. I have this setup and it works great with atmos content everywhere. 

It does require the Arcana and an atmos capable receiver. I also have Sharc from thenaudio. You have to use rf or ir to control volume but audio is perfectly in sync and atmos on both arc and the receiver.