Ok:

First, to change its in battle texture, open up descr_model_battle.txt in Data
Search for: roman_legionary_cohort_ii (I think that was the unit you wanted to add)

You should see:

Code:
type				roman_legionary_cohort_ii
skeleton			fs_javelinman, fs_swordsman
indiv_range			40
texture			romans_julii, data/models_unit/textures/unit_roman_legionary_II_julii.tga
texture			romans_brutii, data/models_unit/textures/unit_roman_legionary_II_brutii.tga
texture			romans_scipii, data/models_unit/textures/unit_roman_legionary_II_scipii.tga
texture			romans_senate, data/models_unit/textures/unit_roman_legionary_II_senate.tga
texture			seleucid, data/models_unit/textures/unit_greek_legionary_seleucid.tga
model_flexi_m		data/models_unit/unit_roman_legionary_high.cas, 15
model_flexi_m		data/models_unit/unit_roman_legionary_med.cas, 30
model_flexi_m		data/models_unit/unit_roman_legionary_low.cas, 40
model_flexi		data/models_unit/unit_roman_legionary_lowest.cas, max
model_sprite		seleucid, 60.0, data/sprites/seleucid_roman_legionary_cohort_ii_sprite.spr
model_sprite		romans_senate, 60.0, data/sprites/romans_senate_roman_legionary_cohort_ii_sprite.spr
model_sprite		romans_scipii, 60.0, data/sprites/romans_scipii_roman_legionary_cohort_ii_sprite.spr
model_sprite		romans_brutii, 60.0, data/sprites/romans_brutii_roman_legionary_cohort_ii_sprite.spr
model_sprite		romans_julii, 60.0, data/sprites/romans_julii_roman_legionary_cohort_ii_sprite.spr
model_tri			400, 0.5f, 0.5f, 0.5f
Now add this line after the last 'texture' line and the first ' model_flexi_m' line:

Code:
texture			greek_cities, data/models_unit/textures/unit_greek_legionary_seleucid.tga
Now add this line after the last 'model_sprite' line:

Code:
model_sprite		greek_cities, 60.0, data/sprites/seleucid_roman_legionary_cohort_ii_sprite.spr
Second, to change the unit card:

go to data\UI\UNITS\SELUCID

And copy the file #GREEK_LEGIONARY.TGA
to data\UI\UNITS\GREEK_CITIES

Done, hope it works.