From a Sabayon Linux Five Oh! Live Session...
>> Installing (1 of 1) x11-misc/slim-1.3.1-r5
The configuration file is located at /etc/slim.conf.
If you wish slim to start automatically, set DISPLAYMANAGER="slim" in /etc/conf.d/xdm and run "rc-update add xdm default". By default, slim will use default XSESSION value set in /etc/rc.conf.
If you want to use .xinitrc in the user's home directory for session management instead, see README and xinitrc.sample in /usr/share/doc/slim-1.3.1-r5 and change your login_cmd in /etc/slim.conf accordingly.