These are not "nicknames". As defined, nickname means, "an invented name for somebody or something, used humorously or affectionately instead of the real name and usually based on a conspicuous characteristic of the person or thing involved. His nickname was Lefty because he was left-handed."

Company names came first; company letter designations replaced them. During antebellum times volunteer militia companies organized under state law and petitioned their representative in the state legislature to incorporate. Being chartered by the state legislature under a specific name, such as the "Darlington Rifles", an act of the state legislature was required to change a company's name. Oddly enough, several companies could share the same name. For instance, at least two South Carolina companies entered Confederate service as the "Darlington Rifles."

Here are some sources on the 8th South Carolina Regiment --

http://batsonsm.tripod.com/b/reg8.html
http://sciway3.net/proctor/marion/military/wbts/SC8thInf.html
http://www.fullbooks.com/History-of-Kershaw-s-Brigade1.html

EIGHTH SOUTH CAROLINA REGIMENT.

The Eighth Regiment was organized early in the year 1861, but the companies were not called together until the 14th day of April, arriving in Charleston in the afternoon of that day, just after the fall of Fort Sumter. It was composed of ten companies, as follows:

Three from Chesterfield, two from Marion, two from Marlborough, and three from Darlington, with Colonel, E.B.C. Cash; Lieutenant Colonel, John W. Henagan; Major, Thomas E. Lucas; Adjutant, C.B. Weatherly.

Companies first taken to Virginia:

Company A -- Capt. Axalla J. Hoole, Darlington, "Darlington Rifles".
Company B -- Capt. Minor J. Hough, Chesterfield.
Company C -- Capt. Wm. Henry Coit, Chesterfield.
Company D -- Capt. John S. Miller, Chesterfield.
Company E -- Capt. S. Mercer Keith, Darlington.
Company F -- Capt. Wm. H. Evans, Darlington.
Company G -- Capt. John W. Harrington, Marlboro, "Marlboro Guards".
Company H -- Capt. Robert L. Singletary, Marion, "Jeffrey Creek Guards".
Company I -- Capt. Eli T. Stackhouse, Marion, "Marion Guards".
Company K -- Capt. Donald McD. McLeod, Marlboro.

After remaining in Charleston until the 4th of May it was moved to Florence. On the 1st of June the regiment re-enlisted for Confederate service. They were ordered to Richmond and arrived there on June 4th, and left on the 15th to join the Second then at Bull Run. On the 22nd of June they went into camp at Germantown, near Fairfax Court House, where all the regiments were soon joined together as Bonhams' Brigade.

Company letter designations changed when the regiment reorganized for the war in 1862. At that time two new companies were added to this regiment, these being Companies "L" and "M".