Things can get kind of messy.

I think really, for KDE, you'd do better to kill two birds with one stone - clear a 20GB space and install Mandriva there and try out KDE in Mandriva.

I heard that if you install XFCE over Gnome there are issues with gnome services. If you use Nautilus, apparently, it will make gnome take over your XFCE desktop - but it can be cleaned up by uninstalling stuff.

Personally, I'd make another install and play with it there first - it's very easy to delete a separate install if you don't like it and it won't mess up your system too much.

If your internet isn't too slow, get a Kubuntu CD for the job.