Installing multiple window manager

[Pierre Girard]

Hello,
     I'm installing an ubuntu machine with the default window manager 
(unity) but I want to also provide kde to users as well as gnome.

For gnome, I install gnome-shell and it works. For kde, I tried installing:
kde-plasma-desktop

I also tried :
kde-full

In both cases, after everything is installed, I reboot and then try to 
login. When I do, I get the kde splash screen, however I never get the 
desktop. All I get is a black background and the mouse cursor.

I tried installing:
kubuntu-desktop

and this seemed to be worst. After the installation, I rebooted but when 
it restarted I got the kubuntu logo and colors (blue) instead of the 
normal ubuntu one (brown) and it never got to the login screen.

Any suggestions on how to get multiple window managers working on the 
same machine?

Thanks.