Lisi Reisz composed on 2015-08-14 00:49 (UTC+0100):
I would expect you to be able to just uninstall the dm (gdm3? kdm?), boot into the command line, log-in as whomsoever you wish, and startx. ISTR that you can put the GUI you want to boot into from startx in an x conf file (.xsession? probably not). I did this once, but it was ages ago and my memory is fading.
In Jessie it here appears that to make TDM the default DM all that's required is to include the following as the only content in /etc/X11/default-display-manager:
/opt/trinity/bin/tdm