Obviously the Play:5 (gen1) uses the S1 App - just leave that App alone and switch off the Play:5 at the power socket.
Download the S2 Sonos App to your ‘compatible’ mobile device and power on your new One-SL speaker. Open the S2 App and after accepting the T&C’s chose the option to create a new Sonos system and setup/add your One-SL to the S2 App - add all your music services you require such as Spotify, Amazon Music, Deezer or whatever you use.
Now power on your Play:5 (gen1) and use the S1 Sonos App to control/play to that speaker.
Simply use the S2 App to control/play to the Sonos One-SL.
Note both systems can use the same Sonos Account login credentials (recommended) or you can choose to use an entirely separate Sonos Account if you prefer (not recommended).
Each system is actually separated by a hidden Household ID reference when setup in the way described above, so there is no opportunity of any conflict between the two systems.