Once you install fvwm2 properly, you can use it. THe "hackerish" way is to just run it from a terminal (I do that anyway) instead of from a Display manager.
First, make a file in your home directory called .xinitrc
Put the following in the file
Now go to a terminal (hold down CTRL-ALT and press the F-key for the appropriate number of the terminal you want).
And "BOOM", you'll be in fvwm2! And you'll still be able to run your regular window manager (if you don't understand how this all works or whatever, you'll have to ask).
Now, you probably want the option to start fvwm2 from the pretty login screen. I've never done it, but I hear that you have to edit the file /etc/X11/xdm/Xsession, adding
exec -l $SHELL -c "$DBUS_LAUNCH fvwm2
under the case "$1" in
part in the file. I don't know this for sure, as I've never done this.
THat should give you a start. There may be much, much easier ways to do this, but I'm not currently working in FC3 so I can't play around with anything to find out.
-- the dudeman