Can we add an Ubuntu derivative to list of computers to manage in Ubuntu Landscape or just Ubuntu?
At least the official ones (Lubuntu, Xubuntu, etc.).
I don't know about support in the commercial sense (as in - will you get paid support from Canonical with them), but landscape technically works fine on derivatives. I personally run Ubuntu/Xubuntu/Lubuntu and all of them work properly.
Full disclaimer: I'm a Landscape engineer.