Recommend for what? Best surround? Best for stereo? Best new speakers to add to existing system? Something else?

I have a surround system using Arc+Sub+2Symphonix. It has worked great.

However the total volume of the system is low, for parties. For day to day usage, the volume/total system output has been sufficient.

 I bought two One SLs to add to this config.

should  I replace the Symphonix with the One SLs in the surround set up (config used most while watching TV or listening to music) and configure the Symphonix as a stero pair for parties? That seems intuitive to me. Is there any other config possible?

why does sonos disallow adding all 4 speakers to one group?

I would personally just leave the Arc, 2 x Symfonisk and Sub exactly as they are, bonded together as an Atmos-capable Sonos Home Theatre with surround sound. Then either use the One-SL’s either as a stereo pair, or standalone, (whichever best suits the room) and ‘group’ those with the Arc HT setup for music playback only.

I would not group the Sonos rooms together for TV audio as you may experience an echo in the room due to the grouped room TV audio buffering delay of 75ms, or greater, especially when having all speakers in the same physical room, but such a setup is fine for music playback and perhaps consider setting the Symfonisk surrounds to ‘Full’ mode for music playback in the Arc room (surround audio) settings.

Personally, I think I’d use the Ones as a stereo pair -call them Music or whatever - and leave the home theatre system as is. Then at party time just group the two “rooms” and you’ll be playing your music through Arc, Sub, 2xSymphoniks and 2xOnes, all synchronised. 

And, Sonos does allow multiple rooms/speakers to be grouped, to a maximum of 32 devices.