You're right that using 2 traditional package managers in the same environment will cause problems almost immediately, since both will want to control the same resources.

This is less so with Nix, as it mostly operates on its own folder called nix-store, the binaries in which are then symbolically linked to. It is actually designed to be able to be used this way. I know people are using it on Mac OS and I intend to install more and more packages through it to later possibly transition to NixOS.