William T. McGrew does appear on records of the 1st Tennessee (U.S.) Cavalry. The regiment served in and around Walker County during March-April 1864. I don't know how Mr. McGrew could fight with Mr. Johnson, since Madison Johnson evidently died during the Battle of Murfreesboro TN, Dec. 31, 1862.

M. C. Johnson appears on the 1860 census of Walker County GA with his wife Elizabeth and two small children. On July 10, 1862, he joined Co. "A", 3rd Confederate Cavalry Regt. at Charleston TN. H. M. C. Johnson of Walker County GA also appears on roll. M. C. Johnson's service record gives his first name as Madison. Copies of both service records are available here.