I have 4 lights, and 1 grenader per stack. Light Infantry are great for putting them up ahead of the main army to skirmish, then fall back as the enemy advances. Once the lines engage, the light inf can be used on the flanks to shoot up the enemy from the side.

Grenadiers are great if a melee develops. I make them throw their grenades into the mass a few times, then charge in if the enemy didn't yet break. Otherwise, if nobody commits to a melee, I just keep the Grenadiers in reserve to plug in a gap the lines will leave once their numbers are reduced.

Try playing multiplayer. People really use these tactics quite well.