NO Gedolim Disagree, what they might say its BETTER for seperate seating
I have been to plenty of affairs with MIXED seating and Gedolim were there.
I have also been to affairs with Seperate AND Mixed seating and while the Gedolim sat in the seperate seating, they stayed for the entire event.
I personally had a simcha (within the last 12 months) where I had mixed seating and someone well known came and knew it was mixed seating (I told them before the event) . His only request was that he sit with his wife and immediate family in a seperate table which I was more than happy and able to oblige AND he gave a Devar torah at this event