The surround system will become a SONOS “Room”. The single ONE will become a second Room. You can “Group” Rooms to play the same music. If you intend to Group TV audio with the ONE, be aware that there will be a delay in the ONE’s output with respect to the TV audio. This will be an issue if you can hear both the ONE and Surround audio simultaneously. If the ONE is in a distant area, you will not notice the delay.

Ah, so two groups and then join the groups for music. Will there be a delay in the music?

I don‘t plan to join the groups for TV at all.

I am basically having an open living area (with the TV setup) and a kitchen area (with the lonely One). For TV, I don‘t want the kitchen ONE to play.

btw. Should I invest into a FIVE for the kitchen or is the setup enough?

There will be no delay for music sources.  The two rooms will play perfectly in sync. 

FIVE is physically much larger than the ONE and can deliver more bass. “Best” is your opinion of the results. When oriented horizontally the FIVE is stereo and will seem more spacious than a single ONE, but unless you are almost nose close to the FIVE there will not be an obvious LEFT-RIGHT to the sonic image.

Will this also work when using Alexa on the Arc to play music?