"Despite the mythology of the keen-eyed sharpshooter, most units were used as scouts and skirmishers. Because they were in the forefront of battles, casualty rates were higher than normal line units. “The proportion of killed and wounded in the sharpshooters was exceedingly large, probably without a parallel. The battalion went into the fight with 104 men and officers, and of these ninety-four men and officers were killed or wounded.” So wrote Confederate Major George Bernard, then commanding a sharpshooter unit during the siege of Petersburg in 1864-65."
A definition of terms might prove helpful.
Scouts find the enemy. What is his strength? Where are the enemy's flanks? Unit patches may reveal the identity of the enemy force. Perhaps the scouts can snatch some stragglers or deserters for questioning.
Pickets prevent the enemy scouts from doing the above. A screen of pickets at night keeps enemy raiders from pillaging your supplies while your main force sleeps.
Skirmishers screen your units' movement. A Division in line charging into battle is quite formidible. A Division in column spread out along a mile of road is the opposite. So a light force of skirmishers moves ahead and to the flanks of the main unit to keep the path clear of the dangers of ambush or snipers.
On the grande scale, cavalry performs this role for an army. However, for the local commander, a group of sharpshooters would be the scouts, pickets or skirmishers. In the game, this is reflected by a +1 initiative bonus to elements in a Brigade or Division that contains at least one Sharpshooter element.
Now to the question of light infantry. A regular infantry unit during the CW may have a number of mule drawn carts with supplies and ammo. The regulars can only go where their supply carts can go with them. Light infantry probably carried everything they needed on their backs. Now say that an enemy formed a defensive line by anchoring one flank on a seemingly impassable swamp or dense forrest. They could be reasonably sure that a force of regular infantry could not pass through this type of terrain. However, a force of light infantry might slip around their flank and unhinge their entire position.
Good luck with your mod ideas!