Well I gave the Celts, Saxons and Alemanni the horde ability in my mod. They don't start out in a horde, but still gain the ability when they have just one city left. I haven't tried to see if they can start as a horde though. Have you made sure in the descr_sm_factions.txt file that the horde units that faction have, actually exist? I had to make new horde units for example.