Yes, you can have as many different graphical desktop environments or window managers installed at the same time as you like (as long as there is space on your harddisk to put the files in). Add WindowMaker, Fluxbox, IceWM and OpenBox if you like.
I believe Compiz Fusion works with XFCE too, at least there seem to be quite a few how-to's and even YouTube videos about it out there.
If you're playing with an older computer, make sure it has enough RAM to run the heavier desktop environments (namely KDE and Gnome), and a decent enough 3dfx card and Linux drivers for it that enable 3d hardware accelaration, because Compiz Fusion won't work without it.
Note that Beryl and Compiz were the "old two", that later became one called "Compiz-Fusion". So you'll probably only want Compiz Fusion now.