Are there any conflicts with installing different Desktop environments in Ubuntu ?
I have already installed Gnome-Shell and can't log-out the session to switch DE.
I have to reboot to change.
I'm not sure what exactly your problem with GNOME might be, but to answer your general question: You can have different DEs in Ubuntu (and Linux in general) without any problems.
Only when using PPAs, you sometimes can run into trouble with library dependcies and stuff.