Well, for Roman positions near Sicily, you run into the issue where you have to pick a faction that doesn't exist anywhere else on the map; you'll thus have to create a new Invader scenario if you want to play as Rome. For Sicily specifically, you ought to be able to just copy-paste-rename the Sicily subfolder in the Scenarios folder, then change the start.lua to have dofile("resources/scenarios/MyScenarioName/Invaders.lua"); at the start and change the part that says "FactionScreen" to say "InvadersScreen".
invaders.lua will take a pre-defined faction name, so if you name something Rome, it'll switch to be Rome, with all its faction bonuses and city-produced unit lists. The starting units do have to be manually switched though. You don't have to make a new category like brigadesgreek, but you can if you think that looks cleaner. How it works is that every invader faction has a part that says brigades=somethingorother, where somethingorother can be a category like brigadesgreek or it can look like the Mamertine brigades={
javelin={name="sabellicJavelineers", cost=1},
javeln2={name="sabellicJavelineers", cost=2, xp=3000},
spearmn={name="sabellicSpearmen", cost=1},
spearm2={name="sabellicSpearmen", cost=2, xp=3000},
lswords={name="sabellicLightSwordsmen", cost=2},
lsword2={name="sabellicLightSwordsmen", cost=3, xp=3000},
hswords={name="sabellicHeavySwordsmen", cost=4},
lhoplte={name="sabellicLightHoplite", cost=2},
hhoplte={name="sabellicHeavyHoplite", cost=3},
cavalry={name="sabellicNobleCavalry", cost=4},
warship={name="heavyWarship", cost=2} }It's the same organization as a category like brigadesgreek, just done right there rather than before the factions table. You can add to or remove from this list as you like, and the codenames on the left can be anything so long as they're unique. There's an army comp below the list of possible brigades that uses the codenames to figure out how many of each type the faction gets by default.
For reference when making your brigadelist, here are the names you'd have to use for each Roman brigade if you wanted them to be an option: latinEquites
latinHastati
latinHeavyHoplite
latinLeves
latinLightHoplite
latinPrincipes
latinSpearmen
latinTriarii
latinVelites
romanwarshipbrigade
flagship
heavyWarship
largeTransport
scoutShip
smallTransport
Sorry this took so long, hope this helped! Let us know if anything is unclear or you have any trouble.
