I'm an occasional Guix contributor. Yes, you can run Guix packages on top of other distributions (GuixSD is a standalone distribution of Guix, whereas Guix itself is a package manager, so it can be used under any other distribution). The Binary installation section shows you how to easily set up Guix on top of another GNU/Linux distribution. You can also run Guix without splatting it over your root filesystem; see the "Running Guix Before It Is Installed" section. (There are other tutorials out there; I've even written my own, you can search for it if you so care.)
So yes, Guix can be run as a userspace packaging system on top of a more "traditional" distribution. (You do need the daemon running as root and the worker users and etc, but once you have that, different users can installing packages for themselves without clobbering each other.)
However, you might notice that maybe it's a bit more work than desirable to get Guix running. It would be much nicer if you could
apt-get install guix or install from yum, pacman, etc. That would reduce some steps! Guix could be packaged for other distributions; Diane Trout was working on this for Debian. However, for good reasons (maybe too long to go into here?) Guix does not follow the Filesystem Heirarchy Standard, and for that reason alone will probably not be installed in the main repositories of Debian at least soon. Maybe some day this will change.
Hope that helps!