I think that all factions should have 'Standard' and 'Special' units.
Standard Units would be Spearman, Swordsman, 'Basic Ranged', 'Basic Mounted', Catapult or Ballista, and 'Basic Caster'.
Special Units would be unique to each faction, like Paladins, Unicorn Riders, Giants, Cannons....
If you took over a factions town you could build all of thier Standard units but not the Special units.
Simple, easy to code (More time for cooler stuff), and it keeps each faction feeling unique.