Your getting the crash because the Byzantines need a texture entry for the ribault in the modeldb file (in SS\data\unit_models). If you are ok with using another faction's skins, then you can just copy the texture entry for another faction.
1. Open the modeldb file in notepad
2. Do a search for ee_ribault_crew
3. Replace the entry you have with the following:
I've marked the changes in red. Basically this just adds a texture entry for another faction.
Explanation:
5 <---the number of faction entries to follow (was increased by 1 because we added a faction. The default SS file has 4 here)
9 byzantium <---- faction name
77 unit_models/_Units/EE_Peasant_Leather/textures/mtw2_EE_Peasant_russia.texture <---- texture path and file name
78 unit_models/_Units/EE_Peasant_Leather/textures/mtw2_EE_Peasant_normal3.texture <---- normal map texture file name and path
47 unit_sprites/russia_EE_Catapult_Crew_sprite.spr <---- path and name for sprite.
The number at the beginning of each line is the number of text characters to follow on that line (the game uses them to know how many characters to look for when reading the file)
The second lot of changes is for the units attachments (i.e. weapons, shields etc).