The two are not related.
For the unit texture, all you need to do is open descr_model_battle.txt, find the model entry for greek archers and add a texture (and sprite) line for carthage, like this:
Code:
type greek_archer
skeleton fs_semi_fast_archer, fs_semi_fast_dagger
indiv_range 40
texture macedon, data/models_unit/textures/unit_greek_archer_macedon.tga
texture seleucid, data/models_unit/textures/unit_greek_archer_seleucid.tga
texture greek_cities, data/models_unit/textures/unit_greek_archer_greek.tga
texture carthage, data/models_unit/textures/unit_greek_archer_greek.tga
texture thrace, data/models_unit/textures/unit_greek_archer_thrace.tga
texture merc, data/models_unit/textures/unit_greek_archer_mercs.tga
model_flexi data/models_unit/unit_greek_archer_high.cas, 15
model_flexi data/models_unit/unit_greek_archer_med.cas, 30
model_flexi data/models_unit/unit_greek_archer_low.cas, 40
model_flexi data/models_unit/unit_greek_archer_lowest.cas, max
model_sprite thrace, 60.0, data/sprites/thrace_greek_archer_sprite.spr
model_sprite greek_cities, 60.0, data/sprites/greek_cities_greek_archer_sprite.spr
model_sprite carthage, 60.0, data/sprites/greek_cities_greek_archer_sprite.spr
model_sprite seleucid, 60.0, data/sprites/seleucid_greek_archer_sprite.spr
model_sprite macedon, 60.0, data/sprites/macedon_greek_archer_sprite.spr
model_tri 400, 0.5f, 0.5f, 0.5f
For the UI, you need to copy the greek archer unit-info card and unit card from *data/packs/data/ui/unit_info/greek_cities and *data/packs/data/ui/units/greek_cities and paste them into data/ui/unit_info/carthage and data/ui/units/carthage respectively.
* In vanilla RTW unit card files are packed inside ui_0.pak and ui_1.pak files (can't remember which one atm), located inside the data/packs folder. You will need to unpack those using pak extractor, so that you have access to the files. After you have unpacked, you'll see a packs/data/ui folder created. You should copy the files you need from there, then go to RTW/data, create a folder named 'ui' with two subfolders, 'unit_info' and 'units', and in each one of those a 'carthage' subfolder. It is there that you put the cards you copied.