in Rifles and Rebels, skirmishers are covered by "unformed" troops. In the case of US SS units it's a good idea to give them high reps, say 5 for most units to show their higher level of training in the light infantry role.
There is a different treatment of skirmishers in Muskets and Shakos that you might find interesting. In that game the skirmishers act with great independence and under rather less control than in the ACW title. In the Napoleonic period though forces suffered relatively high attrition to skirmish lines and commanders had to be careful not to let their attacking force dissipate into one large skirmish line.
By the 1860's tactics had evolved somewhat and light infantry training was almost universal. That is why i opted for the simpler approach in R&R.
Hope that make sense and please feel free to ask more questions.
And Ed, please do feel free to let me know when such questions/discussions pop up here as I do not remember to check in very often.