A:AnswerYes! I have the Sonos Arc, Roam, Sub Mini, and two One SLs, and I regularly use them independently.
Oftentimes, I will have the Arc and Sub paired to the TV in one room with a movie going while the two One SLs are playing music in separate rooms, and the Roam playing a whole other project in another room.
A:AnswerYes, they can play the same thing. They need to be on wifi. You can either select both in the app to play or you can start playing on one of them, then long press the play/pause button on the other speaker(s) to join the stream.