Considering that IRL all the really big guns were mounted into turrets (battleship and cruiser guns, f'rex, but railroad artillery was also frequently turreted) it actually makes more sense that big guns be mounted onto turrets. Anything too big to manhandle into position was mounted on a turret because it was the only real way to aim it.
Even tracked SPG's (the biggest guns usually deployed with land armies) usually have turret mountings these days for maximum versatility. The only exception of any size are weapons that are targeting really big targets (such as the German "Paris Guns" that only had to hit somewhere in a city).
So yes, there should be Large and Huge turrets. The question is should there be Large and Huge gimballed or fixed, because really large caliber guns are almost never deployed in that way.