The regiment was known at various times as Caudill's Regiment Kentucky Infantry, 10th REGIMENT KENTUCKY INFANTRY, 10th Regiment Kentucky Mounted Riflemen, and 11th Regiment Kentucky Mo Regiment Mounted Infantry. The 11th Regiment Kentucky Mounted Infantry became the 13th Regiment Kentucky Cavalry February 22, 1865.
"If they took the oath of allegiance I would imagine that Ruel did too but I can't find any record of it."
The records do not show any of these men taking the Oath. The last records show they were paroled for exchange which precludes oath taking as they would be rejoining their Confederate Army units.
"Here is the record where it mentions "disposition unaccounted for." He is number 9 on the list."
http://www.flickr.com/photos/25565803@N07/8066543458/sizes/l/in/photostream/
I cannot read the document, I assume it's a Federal prison document. I would assume, the clerks lost track of him.