I am not sure if it is safe to install more desktop environments. What versions are safe? I would like to have Gnome and KDE on Ubuntu. Last time I installed Gnome-full has broken my login screen and it was not ubuntu login screen anymore. It was like Gnome login that I saw in Debian.
Installing Gnome 3 (Unity and Gnome Shell) and KDE will result in software from both environments in the Menu, such as the Unity dash and the Gnome Shell applications grid and just as that Gnome Software appearing in KDE's menus.
GTK software will most likely than not appear bad looking in KDE and viceversa, to remedy that you should use the
Nothing will brake, not that I know of. When you install one of the two you will be asked what Display Manager you want to use (GDM or KDM or LightDM).
Also, the vanilla Ubuntu package which contains unity is called