It depends. If you never installed the S2 app then everything should work just fine, as all devices will be on S1 firmware and you have the S1 controller.

However if you have installed the S2 app AND upgraded the Play 5 then you are SOL as now you have a split system.

Orthogonal to this is the “wedding problem” - you will find many other threads on that, but generally it is not recommended to use a Sonos system for life critical events such as weddings due to unexpected wireless issues caused by a lot of warm bodies. For general use in a back yard its fine, and you can easily test this out, when you are not participating in a life changing event.

Assuming the “two S1 speakers” can only run S1, it’s perfectly possible to use them with a Play:5/g2 even if the latter’s been updated to S2.

Just factory reset the Play:5 and add it to the S1 system. It will downgrade itself to S1.

Ok great news. Thank you for your reply