There was an issue with Gnome 3.10 breaking Unity a few months back, but that has been fixed. More in that issue here and here. However, I know of no issues with Unity breaking Gnome.

If you wish to install Unity you can either use the command line(terminal) or the Software Center(can't remember what it is called in Gnome, Munon?)

In a terminal type

sudo apt-get install ubuntu-desktop

in the Software Center search for ubuntu-desktop -search exactly as typed, including the hypen to find it easier.

Either way will download and install Unity and all the programs that go with it....In my system it shows as 9.5MB to download and 32MB once installed but I think that is only the Unity desktop, with all other programs, I would expect it to be near 100MB download, about 300MB after install.

After you install Unity select "Ubuntu" as the DE on login by clicking on the icon in the upper right of the box you type your password in. The menu will open allowing you to select Gnome or Unity.

You can switch between them without rebooting but you will have to log out of one and into the other so you will lose any unsaved info when you log out.