Political situation in Milan around the years 1250-1275:i found out that, if you're plannin of makin the mod start at 1259, you could make the faction leader be Martino della Torre (Martin of the Tower), that were the city's "Captain of the people" (Capitano del Popolo, in italian) and that, after the battle against Ezzelino da Romano (Seignur of Verona), assumed extraordinary powers inside the city...the "Della Torre" dinasty, "reigned" from 1259 till 1273...
more specifically:
at the beginnin of the XIII century, Milan had over 100.000 abitants, and was economically well-organized due to the trades and on the industries.
After the battle with Ezzelino da Roma (1259) lord of Verona, Vicenza, Padova and Treviso, Martino della Torre, that was the city's captain of the people managed to reach extraordinary powers inside the city, inheritated to his brother Filippo, at the time of his death. After Filippo was the time of Raimondo, bishop of Como and Leader of the Lombard League, to lead Milan and the lombard cities from the 1265 to the 1273, thank to the powers he had achieved.
The "della Torre" family, in this way, seemed to have achieved definitely the control of the city, but the ghibelline side, leaded by the "Visconti" family, was waitin for the right moment to strike.
Infact, with the approvation from the HRE's emperor, Henry VII, Matteo Visconti managed to take the power for himself: with him start the leadership of the Visconti in Milan, that will reach his maximum power with Gian Galeazzo Visconti in the 1359.
And here's a list of stendard and banners for Milan:the lombard cross, emblem of the city (only the red cross on the white shield):
war stendard used by the city, and then from the "Visconti" family:
stendard of the "della Torre" family: