I already know that I can uninstall Unity if I'm using Gnome-Shell exclusively, but I'm also wondering if it's safe to uninstall Compiz when I'm only using Gnome-Shell, since as far as I know, Gnome-Shell doesn't use Compiz.
Removing compiz also removes the packages unity & ubuntu-desktop.
Thus you will lose the ability to log into the unity or unity-2d desktops.
ubuntu-desktop is a meta package - normally you would need this to allow a successful upgrade to ubuntu+1. Therefore - just reinstall this package before you next upgrade to 12.04.
On my quick test, removing compiz did not stop me logging out and logging back into gnome-shell. You are correct - gnome-shell does not use compiz as its compositing manager, it uses mutter.
