Yes, with nixos it's easy and _safe_ to install packages from unstable while running a stable system as default (installing stuff never breaks your system).
There's also a fairly new feature called overlays which are used to fully track upstream releases (there's overlays available for rust and Firfox Nightly for instance).
There's also a fairly new feature called overlays which are used to fully track upstream releases (there's overlays available for rust and Firfox Nightly for instance).