You're asking a lot of questions, which is understandable; Wikipedia is not a complete source of information by any stretch of the imagination. Let's begin in the beginning.

After the eighth century, Southern Italy was contested three ways between Arabs, the Byzantine Empire, and "Lombards," who were originally Germanic warriors that intermarried and replaced the old Roman "Senatorial Class."

When Charlemagne conquered Italy from the Lombard King called Desiderius, a rump state survived in Southern Italy; Seeing Charlemagne and his army at the gates of his chief city, Salerno, Duke Arechi of Benevento, a powerful and fiercely loyal vassal of the deposed King of Italy (he had married Desiderius' daughter) came to an agreement with Charlemagne. He sent his son Grimolado as a hostage to Charlemagne's court, and in exchange secured the right to continue ruling as Duke in Southern Italy; Arechi was even recognized the rank of Prince. Although culturally active, the Principality of Benevento would be marred by instability, and by the mid 9th century had split three ways: the Duchy of Benevento and the Duchy of Salerno (whose rulers both claimed the title of "Prince") as well as the independent Republic of Amalfi.

Calabria and Puglia, on the other hand, remained Byzantine Military Provinces (called Thema) up to the Norman conquest. A vestige of Justinian's reconquest of Italy in the sixth century, they were ruled by an ethnically Greek Katepano (Commander) appointed from Constantinople. Because of southern Italy's strong ties to the Greek world, as well as the Byzantine practice of settling soldiers with land, the general picture of this part of Southern Italy is functionally Graco-Latin, leaning more towards Greek at the top of the social ladder, while the bottom would be more Latin.

Sicily, on the other hand, was predominantly under the Abbasid Caliphate by the ninth century. However, when Robert Guiscard (a Norman mercenary employed at the time by the Duke of Salerno) established a foothold in Messina, he revived an old Byzantine title, Strategos, for the governor he appointed; the same title that had been used by the Byzantine military governor (who was based in the same city, no less) when Sicily was originally attacked by Abbasids. Curiously, in the County, and later Kingdom, of Sicily the governor of Messina and its surroundings would continue to be called with the Italianized title "Strategoto", derived from the analogous Greek word, meaning that once the Arab-dominated ruling class was removed a Greek substrate must have been present.

Generally, the early history of Sicily saw Greeks, Lombards, and Arabs fighting with each other in every concievable combination. In 1042, Duke Guaimario of Salerno approved the Norman mercenary William Hauteville's plan to seize Sicily from the Arabs, and in return would be made Count of Puglia (should he be able to conquer that from the Byzantines as well). William (helped by his brother) went above and beyond, and used Puglia as a trampoline to take Calabria, becoming the most powerful person in southern Italy in the process. When in 1052 when Prince Guaimario was murdered, the Hautevilles took advantage of the unrest in Salerno to take the city, completing their conquest of mainland Southern Italy on their way to take Sicily.

William's brother Robert (nicknamed, "Guiscard") managed to impose himself as Count of Sicily. His rule was characterized by the unique coexistence of ethnic Arabs, Greeks, and Normans. There were some limitations on this coexistence, but no real "genocide" as you asked about in your question: in the Norman County (later Kingdom) of Sicily, Greeks and Arab landholders were relegated as "Villeins" (the lowest rank of landholder) and were taxed more heavily than Roman Catholics, a conscious decision which encouraged immigration by Northern Italian Lombards, Britons, Normans, and Provençals, and which rapidly diluted the existing Arab-Greek culture.

Norman Sicily has been defined as a society based on "Unequal Coexistence." Although privileged positions were given to Norman, Provencal, Lombard, and Greek immigrants (more or less in that order) tensions between Christians and Muslims would only come to the breaking point in the late twelfth century. Tensions between Sicilian Greeks and the new "Latins" on the other hand, were easily overcome by professing adherence to the Church of Rome, only recently (in 1054) irreparably separated from the Church of the East, in Constantinople. Indeed, Greek Christians (who had in great part welcomed the Normans) played a key role in mediating between the upper class of Latin lords and the subject populations of Muslim serfs (according to Johns J., The Monreale Survey: Indigenes and invaders in medieval west Sicily). It's interesting to note how within a generation of the Norman conquest, parish censuses show how serfs whose parents have Arabic names take Greek names; a testament to the dominant "Greek Christian" culture in spite of the new Norman ruling class and the almost entirely Arab administrative bureaucracy, and more importantly, the key role of Greek Churchmen in converting the Muslim population to Christianity.

John Julius Norwich, in his The Normans in Sicily puts it this way: “Norman and Lombard, Greek and Saracen, Italian and Jew – Sicily had proved that for as long as they enjoyed an enlightened and impartial government, they could happily coexist; they could not coalesce.” Greek Orthodox inhabitants of Sicily could be expected to be fluent in Arabic, and some Muslims could be expected to be native Greek speakers. However, as with Arabic, Greek language and culture was also rapidly replaced in favor of Latin language and culture in Sicily and indeed all of Southern Italy. If you'd like to learn more, John Julius Norwich wrote a very extensive history of Sicily in two works, one chronicling the Norman Conquest and another on the actual history of the kingdom. Both works have recently been published together as a single work.