Part of the confusion lies in the ADAH record showing service in the 1st and 15th Alabama Cavalry Regiments. Mills was not in the 1st Alabama Cavalry, and there was no 15th Alabama Cavalry. The company that Mills joined on Aug. 2, 1863, was Company C (Dorrance Rangers) a part of Murphy's Battalion of Alabama and Florida Cavalry.
This is a brief overview of the company according to an earlier post by Alan Pitts:
Capt. John W. Murrell's "Dorrance Rangers." Murrell had requested authority to recruit a company of partisan rangers on May 9, 1862. Sixty-six officers and men entered Confederate service under Capt. Murrell on Sept 4, 1862. Murrell reported eighty-one officers and men on roll at Camp Dorrance near Mobile, Oct. 31, 1862, all armed with shotguns and sabres. Camp Dorrance could probably be found at Halls Mill, the company's station on Dec. 31, 1862.
The Confederate Department of the Gulf oragnized Murphy's Battalion of Alabama and Florida Cavalry on June 8, 1863. The battalion included four companies then stationed along the Gulf off Mexico near Mobile. Murrell's "Dorrance Rangers" served as Company "C".
On Sept. 12, 1863, the 15th Confederate Cavalry organized with the four companies of Murphy's Cavalry Battalion, four companies from the 3rd Florida Cavalry Battalion, and two independent companies.
Mills' record can be found filed under the 15th Confederate Cavalry, and you have a synopsis of his service from the letter you refer to. Of interest, in addition to that information, descriptions of him are found twice in his record. The first is from a descriptive list dated "Halls Mills nr Mobile Oct 1, 1863" Here he is described as "Age 18 yrs : eyes blue; hair light; complexion fair; 5 ft. 8 in.; Where born Green Co. Miss.; Occupation farmer." The second is from a register of prisoners of war. It shows that he was surrendered at Citronelle* on May 4, 1865, and paroled at Mobile June 17, 1865. Here he is described as follows: "Residence Green Co Ala.; Height 5 feet 10 inches; Age 19; complexion Fair; Eyes Blue; hair Sandy"
* Although shown on a prisoner of war record, these men were not gathered in camps under guard. They were not present at Citronelle. General Taylor and a couple of aids were the only Confederates present there. After he and General Canby worked out the terms of surrender, orders went out to the various commands scattered throughout the Department of Alabama, Mississippi, and East Louisiana. Federal officers set up parole points in various towns and cities, and men, with their commands or individually, reported to these points to receiver their parole.