I think the way it happens is that in descr_strat, the first unit defined after the general's name and traits with the sam co-ordinates becomes his bodyguards, and generals born thereafter would have whichever unit has the general_unit or general_unit_upgrade attributes. I'm pretty sure that's the way it works but you'd probably be better off asking a member of a mod team such as EB since they would have to know that kind of thing in order to define their own generals' units.