But then doing that would mean that you split a single system configuration through two different tools, right? I don't think it's ideal in practice.
This kind of configuration goes one level above package management, whether it's guix, apt, yum, pacman, or anything else.
Edit: you actually said benefits of guix in the original question - it gives you the way to make a localised installation of something you want. Chef/Puppet cannot be it on their own.
It does, actually. Check out the documentation for details:
https://www.gnu.org/software/guix/manual/guix.html#System-Co...
The fun part is that configurations can be treated exactly like packages and vice-versa. A "package" can depend on a specific configuration too. After you realize that package management and configuration management are not really disjoint ideas, it's the only sane way to think about reproducing software reliably.