Sorry, I don't have any family correspondence between the brothers. The only thing I know about Cyrus is that he went to Georgetown College. This was also about the same time that the future Doctor Samuel Mudd was there. Mudd was expelled for taking part in some type of student protest.
I do have most of Cyrus's military records from the National Archives. There are a couple of letters to Jefferson Davis requesting his promotion to Brigadier General (signed by fellow officers), some prisoner of war records, pay vouchers, and the record of his death.