No

Yes.
The only time they will not be able to play Atmos is if you add a group where there is a speaker that does not support Atmos, then it will play in the best possible format supported by all speakers.

I don’t consider two speakers to be ATMOS. For me, ATMOS is a surround format that requires a surround configuration. However, if you push ATMOS formatted music on a pair of 300’s they will render the music. Here are some details.

I don’t know how Sonos works internally but can’t you assign different “roles” to the speakers while keeping the WiFi / radio identical?

• ERA 300 as Front 
• Arc ultra as surrounds or off

this would “just”be a different channel to speaker mapping in the algorithm?

Thank you for the information.
Do you think adding my submini to the stereo pair still do play Dolby Atmos.

Yes. (But if you already have it, add it anyway. Then look on the Home Screen and you should still see Atmos showing when playing an Atmos source.) 

No problem  you can add a Sub, Sub mini or one of these soundbars: Beam gen2, Arc or Arc Ultra and play Atmos.

You can also add 2xSub (Not Sub mini) but one must be sub 4 and the other must be at least Sub gen3.

I would definitely recommend a Sub 4, but a Sub mini is also a very good Sub. There is just a bit more depth in the large Sub.

That is fantastic! thank you for the information!

You can only have 2 Subs with a soundbar, you cannot add 2 Subs to a stereo pair of speakers.

The statement that one of the two Subs must be a Sub 4 is also inaccurate. See https://support.sonos.com/en-gb/article/set-up-your-sonos-sub