7th Cavalry Regiment
Upon graduating, Lt. Custer was assigned to the 2nd US Cavalry.
Just prior to the Battle of Gettysburg, he was promoted to General at the age of 23 and placed in command of a Brigade of Michigan Cavalry.
During the Indian Wars and at Battle of Little Big Horn, he commanded the 7 U.S. Cavalry.
A regiment is commanded by a colonel. A regiment could be divided into two battalions, or from eight to ten companies.
Colonel MacChesnay (A+)
usually a regiment
Colonel Robert G. Shaw was the commander of the Maine's 54th (colored) Infantry Regiment.
He would command a regiment of artillery
probably a regiment or brigade
The 54th Massachusetts Infantry Regiment was an all-black unit, commanded by white Colonel Robert Shaw
Colonel Banastre Tarleton
The 1st US Volunteer Cavalry Regiment.
This is the famous Massachusetts 54th Infantry Regiment, colored troops. They were led by white officers, Colonel Robert Shaw commanding
Theo Custers was born in 1950.
As a guess, he knew no other commander would volunteer for something that would be a complete slaughter. And, perhaps, as the Colonel of a black regiment, he knew if he didn't volunteer his men, they would be ordered to do it.