I've had a bit of experience running multiple desktop environments, and my best
advice about mixing and matching is, don't do it. If you want to try KDE, I
suggest installing a second hard drive and using it for KDE. No doubt you are
going to hear from a lot of people who will say it worked fine for them, but
starting with 8.04, every time I've installed multiple desktops, I eventually
had conflicts between them. Something always got hosed up. I finally settled on
Gnome. Ubuntu just doesn't seem to have KDE right, yet.