Factions emerging like slavs, via events.txt have to be horde.

If you look at sm-factions you'll see that goths/ostrogoths are horde,
romano_british, empire_east / empire_east_rebels etc. aren't.. I haven't experimented with spawn_on_revolt or shadowing (except to disable them) - so can't say what effect would be of trying to vary that or trying to apply those types of emergence to other factions, sorry.