So if qt clients appear when a gtk compositor already exists, could they use gtk's compositor instead of their own
Basically, "compositor" = "window manager".
If a qt client appears when you're already running a gtk/gnome window manager (metacity), does a qt window manager (KDE) get loaded? No. They all use the same protocol to talk to whichever window manager you're running.
And the existing (compositing) window managers are all expected to become wayland compositors / display servers. Compiz, KWin, Mutter....
So wayland will be just like X, except without X in the way between the clients and the window manager.
Originally Posted by Nobu
is it required (i.e., are two compositors not able to run side-by-side)?
One compositor can run inside another compositor (nested). I suppose you might be able to run two on two different displays. But this probably isn't what you're likely to do.
I've been using E17 as my main desktop environment for almost a year now. I know it's been in devlopment for over a decade (and may never actually see a "stable" release), but it's still a very impressive DE.
Personally, I think it runs great on top of X11. I don't really see any glaring deficiencies as it is, so I'm not exactly waiting breathlessly for a stable Wayland port. But I'm all for progress, and I'm glad to see the Enlightenment developers doing such good work.