Right so this very basics works for configuring all 4 machines, two FreeBSD, one Arch and one Nixos.
You'll notice my idea of Nixos is simply, since it's all a file, to call that file.
This step was about running it all through Ansible. It works. Now the next step is bringing inside the repo, alll my different / broken into parts nix configuration files.
Third step would be to create a common dataset of packages that can be used by the 3 OSs, which should have some equivalency tables since packages are not always called the same across distributions.