Do you still use the word משולש פיצה if it's a rectangular slice, i.e. Sicilian?
Interesting... In that case do you say חתיכת פיצה? If not how would you say a slice of pizza if the slice is rectangular?
In the USA, rectangular slices of pizza are often called "Sicilian pizza". It is frequent as a school cafeteria lunch dish, a packaged frozen dish, and a pizzeria menu option.
In Israel, calling rectangular pizza "Siclian pizza" could seem problematic since it might not automatically be understood as USA rectangular pizza.
I dont think anyone in US says "he eats a triangle of pizza." I certainly dont. "A piece of pizza," yes. If i were talking to my kids i might say "do you want a triangle pizza or a square pizza," but never "a triangle of pizza."
You are absolutely correct, just like an Israeli would never say, "אתה רוצה חתיכה של פיצה?" (Unless, as mentioned above, it's a sqare pizza and there's no other way to say it