I'm no expert but I'll try my best to answer your questions. > Will it know if it should keep/delete/regenerate file manager indexes for each DE for install or

I'm no expert but I'll try my best to answer your questions. > Will it know if it should keep/delete/regenerate file manager indexes for each DE for install or uninstall? Yes > How will it roll back the app launcher icons, which is shared by all DEs? If they are shared, they won't be rollbacked as they are used by other DEs. > Each DE inserts stuff like system monitor, settings etc. Yes? > Will it be possible to install/remove Firefox integration with KDE and Gnome, which is currently packaged by distro? Possibly, as long as the install is proper, it'll rollback everything it did to install it. > If the OS needs Python, can we upgrade Python via PPA so that OS won't use an untested Python? No, PPA is for Ubuntu so it's not relevant here. You'd install Python from NixPKGs, and each tool would declare which version it needs, and you'll have all versions installed locally and each program pointed to the version they need. If you need a different version for a local project, just that local project will use that version. Overall, I'm mainly using AwesomeWM as my window manager. But some weeks/months ago, I wanted to try the full KDE setup. First I tried this on my Arch setup (big mistake) and I still have stray files and things from KDE that I can't seem to figure out how to remove when I wanted to go back. But, did the same thing on NixOS (from AwesomeWM -> KDE -> AwesomeWM) and I had nothing left from KDE when I went back, as I rolled back the entire system to before it even knew about KDE. Pleasure to try out new DE/WMs now.