Generally speaking, the shaping of EDU related to the units of a mod relies on the mechanics of the mod itself, the vision of the developers (how they want units to be), and the type of units existing in a mod...
well, speaking of BC, they are historical, hence no non-human units or fantasy-based, apart a very few "special cases" like elephants...
If you plan to write your own EDU, well, feel free to do that... of course, always rely on a subdivision of troops per tier so you keep balance and realism..