Sergei,
It is difficult to accurately model one culture's increasing influence over another. Even though the latins dominated Sicily, it took hundreds of years for the Arabic and Greek elements to fade away...And to a respectable extent, much of that remained for a very long time past the end date of CK.
At the start of CK, "Arabic" culture more or less dominated most of Sicily with a dominant Greek minority in the east. When the Normans conquered it, they were never a true ruling caste as in Apulia where Normans lorded it over the Greeks and Lombards and formed their own feudal fief systems.
The Sicillian rulers governed more like Islamic potentates then Latin feudal kings. I.E, the focus of the economy remained on the cities rather then on feudal estates which the Norman ruling family never allowed to form. In fact, properly treated, the sizable Arabic and Greek elements of Sicily was a rockbed of stability when compared to Norman Apulia which was constantly rebelling.
Yes, Apulia was more or less culturally dominated but it was far more disruptive and far less productive then the tame provinces in Sicily that enabled it to reach a height of wealth almost unparelled in the Med.
It is import to discern which cultures and nations depended on the strength of the cities and civilian populations and those who depended on the feudal system.
For example, the Italian city-states, Sicily, and to some extent Byzantium (though, the Byzantine military system went through several degradations until it relied almost entirely on mercenaries) the armies were formed from the cities. Consequently, much larger civilian armies were able to be raised. The Sicilians which were basically an island of large populated cities were able to launch an 80,000 man, 400 ship invasion fleet against Byzantium.
England, France, Hungary, the Holy Roman Empire (etc), had to rely on the feudal system in which it would call as many of its fighting class to form an army at any one time. The Holy Roman Empire under the German Emperors were able to amass enormous armies and they were often composed of dedicated, well armoured men providing their own mounts (In contrast to the Sicilians, Italians and Greeks who always used a majority of infantry). These armies were brittle however and only a very strong ruler could keep them in the field even for an entire season. Barbarossa in all his glory and personal magnetism was very hard put to keep an army in the field for longer then six months at a time. The French as well were capable of raising enormous armies, but only for a very short while. They often relied on emotional impetus more then anything else.
England was one of the first "Western" nations to turn towards a civilian army doing away with the feudalistic system or the "snowball" fashion of calling the entire caste to arms and getting it rolling in any one direction.
However, Italian city-states, the Greeks, the Sicilians, etc, and other nations who drew upon the strength of the cities to form their armies could often suffer crushing, debilitating defeats, and still be able to put more manpower on the field.
For the Western nations who were largely feudal based, with one particular mounted class doing the bulk of the fighting, a single crushing defeat could incapacitate their ability to wage effective war for a generation or more!
I also hope that Italy is distinctly represted as a wholly different culture then either their Gallic or Germanic cousins.
Sicily will be difficult to model correctly, yet I hope that culture does not determine tax profit. Having a different culture can even improve stability as a whole. Remember, that it was the Sicilian ruler's ability to draw upon sizable Arabic (they called them 'saracen') numbers as troops that ennabled them to wage successful war against the Papacy even when excommunicated.
Count Roger actively discouraged any attempt to convert his Greek or Arabic subjects, simply because, it gave his realm added stability without it being subverted by the Catholic church.
