For TV audio, the best listening experience is to have the One SLs as surround speakers with the Arc/Sub.
All four Sonos devices should show under one room in the app; Arc room name (+Sub+LS+RS)
To achieve this, go into the Arc’s room settings and select Add Surrounds, if you haven’t already.
For music listening, the best listening experience is subjective. What works for one person may not be the preferred option for another. However, with Sonos it is fairly easy to try the different options and decide yourself what suits you best.
When the One SLs are being used as surround speakers, you would have to go into the Arc’s room settings, select Remove Surrounds then select to move them into a separate room to stereo pair them.
In reality, this isn’t a practical way to use the system each time you wish to listen to music, as you would then have to re-add them as surrounds again for TV audio.
The simplest option, is to keep the One SLs as surround speakers with the Arc/Sub then adjust the Music Level slider under the Arc’s room settings for Surround Audio. Here you can also adjust the Music Playback option between Ambient and Full. When set as Full the One SLs will output the same overall sound as the Arc. The music will also benefit from the Sub in this setup.
For many, they are happy listening to music with the Arc/Sub/Surrounds combined, however, if music listening is important to you, you may want to consider purchasing additional Sonos speakers to be a stereo pair purely for music listening.