I've never done southern genealogy but I am currently working on a 3-generaton event-based genealogy for ~260 men in the union army (259 served from Maine and 1 served from Wisconsin). I took a look at your problem and, even with a knowledge of the data available online, I was overwhelmed by the number of "Stephen D. Smiths" I did find that he enlisted as "SD Smith" and, since I couldn't find him on the 1860 census for Cherokee County, assume he went there to enlist.
Going back to a basic point, how do you know that he is your ancestor? Heresay? Genealogical tracking? It is a basic rule of genealogy that you work backwards, do not go on to the previous ancestor until you have proven the current, and never (never, ever) skip a generation in the same line. It is estimated that less than 5% of genealogical information available is online--the other 95% comes from slogging through courthouses, libraries, historical societies, probate & land records, etc. etc. Even so, you can do a summary job (and fill in credible citations afterwards). I'd hit the obituaries, starting with your parent, then his parent and so forth. Once you've tracked him back, you'll have an approximate date of birth and perhaps a spouse. Then, as the Brits say: "Bob's your uncle"!
Your 3-generation event-based genealogy sounds interesting! If you have a thread on this site about it, please point me in that direction. I'd love to read more about it.
Yes, the fact that there are so many "Stephen D. Smiths" has proved aggravating to say the least. With such a common last name, that is to be expected, I guess. I won the genealogical lottery having my father's side as Smith and my mother's side as Jones! I have a bottle of Excedrin handy any time I begin research.
Thanks for the reminder on good genealogical research practices. You listed several of my pet peeves. I KNOW that this Stephen D. Smith is mine because I followed those practices. I started with my father, then through obituaries, marriage certificates, census records, death certificates, courthouse records, wills, etc. I worked my way backwards. I never add a generation going back until I have at least three "proof" documents. I'm confident in the information I have gathered so far - I've just been stuck not being able to find any more on him for the last several years.
I didn't include all of his biographical information in my initial post as the only question I was concerned with at the time was how rank was assigned at enlistment. I also didn't expect everyone to help by trying to look him up! Thank you for that. I'll include some of his information below, now.
Stephen D. Smith
born: May 1838 in Georgia
died: June 25, 1903 likely Coosa County, Alabama
burial: unknown
obituary: paid for a search at the AL archives and nothing was found
death certificate: he infuriatingly died 5 years prior to Alabama requiring these
wife: married Rebecca Garman in 1885 in Calhoun Co., AL (Rebecca was born in Milton County, GA in 1858. This is a neighboring county to Cherokee so it's possible they knew each other previously in GA even though they married in Alabama. However, he is not listed in the censuses of Milton County.)
military service: Company D, 14th GA Infantry, enlisted on July 9, 1861 in Cherokee Co, GA as 4th corporal, captured at Spotsylvania and sent to Elmira prison camp in NY
known children: Stonewall Smith (nicknamed "Tige") born 1875 from a union prior to Rebecca, Martin Samuel Smith, Ida Smith
possible children: Henry Chester, Luther Thomas (These MAY belong solely to Rebecca from a prior union.)
census records: 1900 Coosa County, AL, Rockford precinct
miscellaneous: Ida Smith's death certificate lists her father as "Dee" Smith. The informant on the death certificate is unlisted so I'm taking this with a grain of salt, but I'm treating this as a possible nickname for Stephen until proven otherwise especially since I don't yet know what the D. in his name stood for.
I plan on visiting the little town of Rockford, its courthouse, its cemeteries, and its library some time this year. Maybe some clues will be uncovered then. Thanks for all your help!