I think the issue at hand is not necessarily the 7-man-upgrade, but the ability to build sandbags (and way better sandbags than the normal ones at that). The problem aren't the 7 men per se, as they are still cons, but it's 7 men in heavy cover when you have to push plus their insane utility with molotovs and AT-nades.
In conclusion, maybe upgrading them to a 7-men-cons should lose them the ability to construct sandbags or something along that line? What do you think about that?
I'm pretty sure the 7th man adds a veteran squad leader bonus that gives them cover buffs like the British. Could get rid of that if it's too much.