Not at all an expert on Fedora (I installed it for a while but replaced it with Debian). However, the problem is how to configure GDM.
When I start up GDM, there is a button someplace, it depends exactly how its configured where it is, which lets you pick the session. It should be something like Options > Select Session, and on my version of GDM its down at the bottom left of the screen. This should show you all the different WMs you have.
When you pick the session option you want, it should then ask you whether you want to make the new session your default or not.
But I'm left feeling that it cannot be this simple a problem or you'd have seen it at once. Is it maybe that when you install the other WMs they are not finding their way into the Sessions menu? That seems very odd. As far as I recall, it worked fine with my short lived Fedora install.
Peter