You can group speakers (https://support.sonos.com/en-us/article/group-and-ungroup-rooms, so you could even use one Era 100 (or a Sonos Port) and group it with the Arc etc., or designate the Arc as it’s preferred speaker.
For pure stereo listening I’d look at the Fives too. If streaming Atmos is likely at some point then the 300s are your best option.
Having multiple Sonos Rooms in a single room of your house is rarely an issue, you can listen to just one Sonos Room or Group them as needed.
I find for music I prefer a Stereo Pair but for background music I like all the Sonos Rooms in my room active. I also set any Surrounds to Full Mode. It gives a very even volume level everywhere in the room. It does disrupt the imaging but that is rarely possible to do well for background or party settings.