Pete Kendall was the L Guard who played between them, 10-11 yr veteran guard who had started his entire career, while Brandon Moore, at his 3rd year as a full- time starter, played the other guard spot. This during the rookie season of Ferg and Mangold.
So don't you think a rookie G would be fine with the veteran leadership of Mangold & D'Brick, especially if they're playing between them?
I do not think a rookie OL playing between 2 veterans is an issue. You asked who the veteran leaders were for Mangold and Ferg. The answer is Pete Kendall and Moore.