If you set it up as a HT 5.1 system you can also stream music to the room, but the front and rear speakers will both/all play stereo sound. It is, at first, a bit odd having the sound coming from behind you if like most you are more familiar with speakers only in front of the listening position. But that’s how my system is set up in my lounge (with Playbase rather than Arc) and I’m now used to it. I have other speakers in other rooms too. 
 

You could set up a 3.0 or 3.1 system for the tv and then use the One SL’s as a stereo system with the speakers placed more traditionally. 
 

It’s quite easy to try both options and hear which you prefer: Sonos makes it easy to reconfigure setups, though it wouldn’t really be practical to do it on a day-by-day frequent basis. 

So in other words if I want to stream music from Spotify connect to The ones and sub I have to reconfigure the HT 5.1 setup to two different groups: Beam as stand alone and the ones and sub as a group?

What if I get two more Ones as front speakers. Can I have two Sub at the same time in a group with the front SLs for music and also in the HT5.1. Or do i Still need to reconfigure every time?

Sonos identifies “linked” speakers by labels, which they call rooms. A room such as Lounge could have a 5.1 HT system in it. You can call the room anything you like: it’s just a label so that speaker set could be called Movies. In the same physical room in the house you could have a pair of One SL’s and label them Stereo. A Sub can be added to Stereo too so in the same room of the house you have a full 5.1 system and a 2.1 system  

You can stream music (Spotify, Amazon etc) to Movie or to Stereo. And rooms can be grouped, to play in sync across multiple rooms. What you can’t do is stream to just the rear speakers in a 5.x setup. 

Thanks. That clears it up. If I decide to go the Sonos route I will have to get another stereo pair for the music listening, either the ones or wait for the new Era 100.

There is a “balance” volume control in the 5.1 config, so you can minimise sound from the rears when listening to music, and still have that music in stereo from the Arc and Sub. Not having an Arc I don’t know how good it sounds as a stereo music speaker. I suspect the sound stage is very compromised as it is with pretty much any soundbar. 

You might consider your setup, Arc, Ones and Sub for 5.1 TV use and then add a pair of Fives for stereo listening.

You can place the Fives in the proper relationship to your seating for an optimal stereo image. You might not feel you need a Sub with the Fives but you can easily move the TV Sub to them and see what you think.

You could also pull the Sub and Ones from the TV and try them as a stereo setup before spending any more money.