hi Daniel.
the joys of tweaking and breaking stuff
- gnome (aka gnome-shell) is gnome3;
- gnome-classic (aka fallback-mode) is gnome3;
- cinnamon depends on gnome3;
- mate is independent from gnome3 (in fact being gnome2 renamed can co-exist in the same installation, or be removed safely)
- twm shouldn't have any deps on gnome3 (never tried it) but i believe is a very small install and you probably going to need a stable fallback system for when testing goes unfrozen (june next year, after debian7 release)
with this in mind (and knowing that you want to keep cinnamon and mate), you can (if you really want)
- uninstall gnome-session-fallback (this way eliminating the gnome-classic entry in gdm3) but if you have an issue with your graphic-card drivers this is the safest session to have (together with mate)
- there's ways to unistall mate (and it's both safe and efficient) but that's not what you want
- cinnamon can also be unistalled safely (it depends on gnome but has not the other way around) but again you want to keep it
just don't get to excited

unistalling stuff because that is one of quickest ways to break the system.