No. Not as a single Sonos ‘room’.

The maximum Sonos speakers in a single ‘room’ is an Arc/Arc Ultra , with a pair of surrounds , and up to two Sonos subwoofers. Any other ‘speakers/Amps’ are required to be in another ‘room’, which would have a 75ms delay on the input to the Arc from the TV, although when streaming music, would be in sync. 

If you are using the Amp to provide front Left/Right for your Arc, that is not possible.  The Arc, like all soundbars, already contains the Left/Center/Right fronts and cannot be used as a standalone Center speaker.  Similarly, the Amp also cannot be used as Atmos speakers.  The Atmos speakers are already in the Arc and Era 300s.

Could I use the amp to power 2 ceiling speakers? They would be used in between the front arc and back surround 300s. Also which would be the better placement for the 300s? On speaker stand or side walls near the back or back wall. Thank you for the response.

Could I use the amp to power 2 ceiling speakers? They would be positioned between the front arc and the back 300s. Also which would be the better placement for the 300s, on a stand in the back, on the side wall near the back, or on the back wall? Thank you for your response.

No.  The maximum number of speakers you can connect to a Sonos soundbar are two surrounds and 2 Subs (only 1 Sub Mini).  There is no way to have multiple discreet surrounds/Atmos/Fronts.  

The best placement for the Era 300 surrounds is behind and to the sides of the main listening position.  Not too far behind, because the Atmos channels need to be able to reflect off the ceiling and their angle is pretty much straight up.  

I saw some videos where the 300s were mounted upside down to reflect off the floor. Is this a good idea?

Also I have epoxy floor no carpet. Thank you

Atmos is a signal meant to come from above, so reflecting off the floor is not what you want.  Mounting them upside down means they are firing directly at the listening area, not reflecting.  But that is only if you can’t mount them the regular way.

I thank you for your answers and help