Gnome-newbie; about Gtk/Gnome-themes

[goblin]

I recently switched from fluxbox (temporarily, just wanted to try Gnome) and I would like to have som clarification about the themes (and some other stuff), I've spent som time around the 'net trying to figure things out, bu to no avail...

What is the difference between a GtkX theme and a Gnome-theme?

Is a metacity-theme typically embedded within such a theme?

Can you exchange the default metacity window manager for another window manager, say fluxbox? Can you do it globally?

How do I install a new GtkX/Gnome theme?
(I have tried using the in-Gnome theme manager but the new themes doesn't show up, a refresh-issue? I have noticed that my ~/.themes folder is being filled with themes, how do I get them to work?)

And, to finish up, how do I set a global default window manager setting for GDM, so that I can remove the available session choices (I don't want my users to be able to choose).

Please help, thank yous, any little help or fragment of answer is most appreciated