Edit /etc/rc.d/rc.4 and insert launching for your requested display manager at the beginning.
It may look like then
# Tell the viewers what's going to happen...
echo "Starting up X11 session manager..."
if [ -x /usr/local/bin/wdm]; then
exec /usr/local/bin/wdm -nodaemon
# Try to use GNOME's gdm session manager:
if [ -x /usr/bin/gdm ]; then