I don't know that your average guerilla would get a significant bonus from fighting in rough terrain in melee when compared to a regular linesmen, unless they had some specific advantage in terms of their armament. Axes or hatchets, for example. A guerrilla with a musket against a linesman with a musket, even in broken ground, would probably see the linesman win, unless the guerrilla also had training in how to fight in close quarters with a musket.