My experience of GNU Guix is using it as a package manager on a 'foreign' distro like Debian. For standalone apps it works pretty faultlessly, allowing me to easily install new version of packages like blender, krita and inkscape and have all of the advantages of guix at my service.
My experience of trying to install things like Python via Guix were frustrating: I often seemed to struggle to import modules, and in the end I went back to conventional installation methods.
Python from Guix has been extended to respect the GUIX_PYTHONPATH variable. If a module you installed via Guix isn't found in your Python from Guix then that's what I'd check. Guix sets this variable (via the generated etc/profile) when you have Python installed.
One thing you shouldn't do or expect to be able to do: mix Python modules installed with your system's toolchain with those installed via Guix.
My experience of trying to install things like Python via Guix were frustrating: I often seemed to struggle to import modules, and in the end I went back to conventional installation methods.