I do not think you can have a X server running but not a X display manager.
The X display manager does not only take care of the login screen:
https://en.wikipedia.org/wiki/X_display_manager_%28program_type%29
You may however want "nodm", a 31.8 kB package present in Trisquel's
repositories where it is described as follows:
This package prepares the system to automatically start an X session at
system boot. It is meant for devices like smartphones, but can be used on
a regular computer as well, if the security implications are acceptable.
Otherwise, you may want to take a look at Qingy (in Trisquel's repository as
well). It is lightweight (the framebuffer displays the login screen!) and
autologin is possible.
Notice that gdm and kdm propose autologin too. Theirs respective dependency
lists are long... but you may have to run them anyway (if you use a Gtk-based
DE in the case of GNOME, a Qt-based desktop in the case of kdm).
