There`s a lot of text and I haven`t covered it all, but to make it short. If you only want to change the unit`s aspect and both units have the same weapons you will only need to edit the DMB(descr_model_battle) and change the texture and model. And the sprites if you have those too. You don`t need to edit the unit`s name or add another unit or change the stats in EDU.
If the DMB text you`ve quoted above is accurate you should end up with something like this:
(it`s not my fault the text is all crowded; next time use the CODE to wrap the text in it; that way it keeps its original aspect).
Code:
type greek_archer
skeleton fs_fast_foot_archer, fs_fast_dagger
indiv_range 40
texture macedon, data/models_unit/textures/UNIT_GREEK_ATHENIAN_MARINE_ARCHER.TGA
texture seleucid, data/models_unit/textures/UNIT_GREEK_ATHENIAN_MARINE_ARCHER.TGA
texture greek_cities, data/models_unit/textures/UNIT_GREEK_ATHENIAN_MARINE_ARCHER.TGA
texture thrace, data/models_unit/textures/UNIT_GREEK_ATHENIAN_MARINE_ARCHER.TGA
texture merc, data/models_unit/textures/UNIT_GREEK_ATHENIAN_MARINE_ARCHER.TGA
model_flexi data/models_unit/unit_greek_athenian_marine_archer_high.CAS, 15
model_flexi data/models_unit/unit_greek_athenian_marine_archer_medium.CAS, 30
model_flexi data/models_unit/unit_greek_athenian_marine_archer_low.CAS, 40
model_flexi data/models_unit/unit_greek_athenian_marine_archer_lowest.CAS, max
model_sprite thrace, 60.0, data/sprites/greek_cities_greek_athenian_marine_archer_sprite.spr
model_sprite greek_cities, 60.0, data/sprites/greek_cities_greek_athenian_marine_archer_sprite.spr
model_sprite seleucid, 60.0, data/sprites/greek_cities_greek_athenian_marine_archer_sprite.spr
model_tri 400, 0.5f, 0.5f, 0.5f
Now, don`t forget to place the models, the texture and the sprites(both the .spr and the .tga.dds files) in their correct folders.
Note: you were missing some sprites lines from the text you`ve quoted. Not to mention the model_tri line. Are you sure you`ve posted the whole section?
Also, the SPQR entry has textures for other factions. If those has units using this DMB entry their unts will all look the same. If you want only one of them to look differently(like the one in XGM) you`d need to make another DMB entry and change the soldier line of the EDU entry to point to it.