Civil War Soldier vs Original. Sounds like a good alternate designation. Maybe even Civil War Original.
General guidelines might be appropriate uniform using regulations, other documentation, and photographs. I would think that as long as the uniform, firearms and accoutrement fit within the 1861-65 time frame, and could be documented for the judges, that it would be appropriate regardless is US, CS, State Militia or foreign observer.
The biggest issue I see with this is that there was considerable variation between regulars and volunteers, among various state raised units, regulations vs practice, and the like. There were probably a couple of thousand ways to wear a slouch hat for example, and various colored shirts or pattern shirts were worn alongside white shirts. It might be necessary to state that it either follows regulation or can be documented with other documentation and photographs. This latter requirement would actually be essential for "guerrillas" to be in the category since there are no regulations for irregular type units (with the possible except of partisan units such as Mosby's that were required by law to wear CS uniforms).
Not saying it can't be done, just that it would require precise definition as to what was required to shoot in the category (whatever it might eventually be called).
US Scout
Bvt Col of Marines
Chief Judge and Bottle Washer