I configured my system to do an autologin using awesome wm. However, after an upgrade of my Debian Sid (which wasn't without difficulty because of a bug in the package of my graphic driver, I had to reinstall some packages such as SLiM before figuring out a solution.), SLiM is now using Gnome Classic as default WM.
My configuration is the following, feel free to ask me more information, I'll provide anything I can.
Relevant parts of
login_cmd exec /bin/bash -login /etc/X11/Xsession %session sessions awesome,startxfce4,openbox,ion3,icewm,wmaker,blackbox default_user max auto_login yes