Only two surround speakers can be added in a Sonos home theater setup. So in your case, if you added the Era 300s as surrounds, the in-ceiling speakers could no longer be part of the Arc setup.

No, you already have the maximum number of speakers in your Home Theater setup. You could unbond the Amp driving those ceiling speakers from the Arc, so that it’s no longer used as a ‘surround’ driver, use the ceiling speakers for music only, and bond the Era 300s as the surrounds for your Arc. 

Thanks so much for the response. So now my only option is to listen to Atmos only with the arc and the Era 300s, without grouping the Amp and then when listening to music in stereo grouping the In-Ceiling Amp.

I think its kind of bad news for me because it seems that I wont be using the In Ceiling anymore.

Anyways thanks for the answer!!

Do you really need the Era? How's Atmos with the in-ceiling?

To be honest im not getting the results I was expecting, the image seems a little diffuse, I own a pair of Sonos Five and sometimes I put them in the rear wall and de activate the In Ceiling for the rears, and I get much better rear speakers results with the fives, im not sure if the fives being at ear level that make the difference. But they are way better than the In Ceiling, that’s why I git excited to hear about the new Era speakers.

So for those of us with only an Arc and sub, the Era series is perfect if I understand the feedback correctly.