Is unity just a plugin of compiz
Solution 1:
The version of Unity that will be released in 11.04 is definitely implemented as plugin(s) in Compiz. (I've seen it with my own eyes...) This is described in a little detail in the blueprint on Launchpad.
It will also certainly be possible to use convential GNOME or the GNOME Shell, though perhaps not in the manner you describe disabling the plugin. It is more likely to be an option at the login screen (if you have it installed).