I went to my favorite info source, civilwardata.com, and searched the Confederacy for all with the name "McCann". 237 were located, only 6 held the rank of Captain. Four were infantry, 2 were cavalry (the more likely connection with Gary's unit).

"Capt. McCann" in the CSA

J.D. AL 25 Inf Co G
J.R. TN 11 Inf B
James A. LA 11 Inf C
James Munroe VA 31 Cav C
VA 17 Cav B
VA 19 Cav Jackson's
Joseph D AL 6 Batt Inf B
Thomas KY 8 Cav A

As for the SC 7th Cavalry, my surnamed great-great grandad was a noncom, his older brother a lieutenant, and a brother-in-law a private. I have skimpy data about the SC 7th; I researched my files, and find no mention of a McCann. This also included a search of the six companies supplied to the unit from the Holcombe Legion.

"The Falling Flag" mentions the units in Gary's command as they left Richmond. SC 7th Cavalry; Hampton Legion Cavalry; 24th Virginia Cavalry; plus a partial company of the Georgia 7th (the only dismounted unit).

Good luck with your search!