On login you're able to select a "workspace" which basically lets you switch between different desktop environments on login. Similar to how you can select an X11 or Wayland session on login.
Layering desktops is probably not a good idea, we do recommend against it. I find layering Cosmic acceptable because I believe it shouldn't conflict with existing desktops on your system and it's small enough to not too significantly change update times. But I won't guarantee this, so do this of your own will.
To install it: copr enable ryanabx/cosmic-epochcopr enable ryanabx/cosmic-epoch rpm-ostree install cosmic-desktoprpm-ostree install cosmic-desktop And then it should just show in your session selector