Yes, You can install different desktop environment on the default system. For example, installing
kubuntu-desktop package on Ubuntu system.
The difference is --
When you install
kubuntu-desktop package on Ubuntu, it will download all the packages of the kubuntu CD. so, more than one web-browser, office software (such as Abiword, Gnumeric installed with XFCE), chat software, partition editor, terminals and etc. So if you want a cleaner system, a clean install with only the preferred Desktop Environment is recommended.
But you should be aware of the fact that you need to install your necessary software on each installed system. So, this method is a bit expensive.
You should also aware of some incompatibility issue of firefox font-rendering between KDE and GNOME system. also
exo-utils related workaround which occur with XFCE installation. Search Google and Ubuntu-forum for more information about these issue.
Note about Integration
I recently installed
kubuntu-desktop in the default Ubuntu. The first point I noted is, The smooth integration between default Ubuntu and KDE. Also KDE more memory friendly now.
My Suggestion is Install Ubuntu and Kubuntu as separate system if you prefer smooth Integration. You can install
xubuntu-desktop on the Ubuntu system. Because these use most of the Gnome libraries. But I installed
kubuntu-desktop on top of Ubuntu, and I think this is good now.
You can try only the Desktop Environment without installing many more extra softwares by using
--no-install-recommends switch to
apt-get command line. For example
sudo apt-get install --no-install-recommends kubuntu-desktop. Kubuntu
sudo apt-get install --no-install-recommends xubuntu-desktop. Xubuntu
sudo apt-get install --no-install-recommends lubuntu-desktop. Lubuntu