Incorrect. It was majority Christian until the appearance of the Ilkhshidids, a dynasty in the early 10th century. being Turks, they were a tad more zealous in their conversions. While the Fatimid state did breed a state of tolerance the Coptic Christians were whittled down by the continuing Jizya. The Mamluks bred a more aggressive conversion technique but the Copts had been the plurality since the 10th century.
What is your source for this? Every history of the Copts I've ever encountered says the tipping point, where Egypt became majority Muslim, occurred toward the end of the 12th century, not early in the 10th century. You're revising the timeline by almost three centuries.
That being said, as far as cooperation with the Copts by the Crusaders goes, it's unlikely. The Copts were and are Monophysites and considered heretics by both the Eastern and Western churches at that period. Historically, the Crusaders tended to treat the Monophysite Christians they encountered about as badly as they did the Muslims under their control...a great many people killed during the great Crusader massacres which occurred at various times during the history of the Crusades were local Christians and not Muslims or Jews. It's difficult to see how they are suddenly going to "embrace the heretics" when they didn't do it anywhere else.
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