Is is possible to use packages from guix repo right in nixos? The same question for the opposite situation: how to use nixpkgs packages in a guix machine?
Both Guix and Nix have procedures for installation on non-home GNU/Linux distributions. And the paths they want are different. In particular, you can install Nix on GuixSD or Guix on NixOS. Or both Nix and Guix on Debian.
Mixing Guix packages and Nix packages as dependencies in either direction is not likely to be feasible at all, as far as I understand. (It is guaranteed not to be easy)
As for the how, at least the nix on guix way will simply be following the normal nix (not NixOS) installation guide: Supported Platforms
That should work for any Linux distro, so you can even have nix, guix and Debian packages all at the same time
If that doesn’t work out of the box, it’ll be due to something like interpreters not existing for the precompiled binaries or such - since those are only used for the initial bootstrap, you can work around that using patchelf, or just build from source (which is also documented in that doc section).
As a word of warning, some, especially graphical packages, can struggle with the underlying system not matching up with correct libmesa/kernel versions and such, but that is to be expected with a frankendistro.
You’ll probably be better off asking the guix guys how to install guix on non-GuixSD platforms, but there is bound to be a similar doc for that too, including the exact same caveat.