Neither lolIf I'm trying to take up some room at the toebox or trying to stop the liner sliding forward in the boot, should I add some foam on the inside of the liner at the toes or on the outside of the liner?
Not at the toes. There shouldnt be ANY room between the outside of your liner and the shell - the liner should not be free to slide.
To prevent your FOOT from sliding fwd... you add foam to the tongue. Can be inside the liner or between the liner and shell/tongue.