I've enabled the root account by setting a password for it but not matter what I try I cant log into any DE but the default unity one, while any other user (but root) can choose any DE
You can't log in to the GUI as root as it is an astronomically bad thing to do. Log on as a normal user and use
DO NOT LOG INTO GNOME AS ROOT
This is impossible to do because if it did allow you do to do that MANY things wouldn't work correctly.
Any reason you might have for wanting to do this stems from misunderstanding something.
For a discussion about why not to log in as root, see Why is it bad to run as root?