Here's a graph, it should be fine for your package needs: Graph
This is not totally accurate because nixpkgs also packages some packages that wouldn't be in the system package manager like Python and Haskell packages. Excluding those it's pretty much the same as the AUR
I did on my Nix, there was a package in Nixpkgs that was outdated, so I had the opportunity to use distrobox for that, at leqst temporarily until they update the package.
You can install Distrobox on Fedora (or any of the distros that support it), create a Debian distrobox on your Fedora install, and within the Debian distrobox you can use apt-get to install whichever Debian package you like. Or..., you could make an Arch distrobox and even install stuff from the AUR. Or really any package from any of your favorite distros as long as it's supported.
And it’ll be segregated from the base system and from other containers, like toolbox installs are?
Exactly. It's even possible to segregate it beyond what Toolbx has been able to do (at least since the last time I checked) in that you can define another folder/directory as your HOME directory within the distrobox.