It is completely isolated from other package managers. So you can use them in harmony, and if you decide that you don't like Guix there's like 2 directories to delete.
I assume unless you use software which installs it's cache/other stuff into the user home directory, right? Then you would also need to delete these dirs as well?
The only thing that ends up in the user's home directory is a ~/.guix-profile symlink.
Software like that exists (e.g. ibus, which creates a database and cache in the home directory), but generally I haven't had any problems with Guix on other distributions.
Ah, good to know. Just tried sbt (build tool for scala) with Nix (have no experience with Guix, but like that everything is in the same language) and it indeed creates .ivy and .sbt caches in the home directory. Everything else goes into the store like it should.