ARC is not compatible with S1. Check for compatibility because you may be able to convert your older units to S2.

For instance, I run PLAY:1s as surrounds for two separate Arc devices, all running S2.

Just update the two PLAY:1s to S2, and set them up in separate rooms, temporarily, then you can ‘add surrounds’ to the Arc. 

Note that Sonos has said that PLAY:1s are not compatible as surrounds for the Arc Ultra. I’ve seen people being successful in that setup, and others not, it seems. I probably wouldn’t try it, as even if it works right now, there is no guarantee that it won’t stop working tomorrow. Much less hassle to use ‘approved’ speakers, while there’d be a slight pain to the wallet temporarily, there’d be no chance of them stopping working. If I were to update to the Arc Ultra. Fortunately, as I run just the Arc, I’m not worried about it. 

Thanks all, after I disconnected the bridge and reset the play 1’s they showed up in the new Sonos app. Then just added them to the arc as surrounds. 

Ah, the Bridge. That is S1 only and probably kept the Play:1’s on S1 too.