Yup, you can do that. Call the One SL’s “Music” and run them as a stereo pair. What you can’t do is add them to the home theatre room. Arc with two Era100’s as surrounds and Sub gives you your 5.1 system. 

I sorta of found a work around. If I open in the Sonos app iOS the gray box that shows what’s playing, I hit the volume slider and it showed the One Sl pair it let me switch them to tv as well! 
  Now there are three slider one for the One SL’s, one for the Arc, Sub and Era 100’s and one for both together. I can have one set louder than the other or same volume, whatever remote I’m using for that source will only control the main Arc Era sub set. Have to use the app to control all at once. 

What you’ve done is use a Sonos feature called grouping. It lets multiple rooms play the same source, and as you say it also lets you set the individual room’s volume as well as that of the group. 
 

With an audio input the grouped rooms will play in sync. With tv input there’s a small delay (70msec) to the rooms not connected to the tv itself. This may be heard as a slight echo. That said, I often group my kitchen with lounge so I can hear the tv audio whilst cooking - I’m far away enough I only hear the sound from one speaker set or another. There is a place where I can stand and hear both - together with the out-of-sync echo - but it’s not a “natural” place to stand so it’s not an issue for me.