That makes more sence that a 1st Sargeant would, rather than just a private, be promoted to the rank of Captain.

Selection of Officers for command of U.S.C.T. was a somewhat questionable process. Few whites were willing to accept such a command, even if it meant a quick promotion to much higher ranks. The fact that you now know of his connection to the 1st Mississippi Marines also fits into the pattern of transfer and promotions. It also gives you a possible new service record to examine for information if you have not done so.

You might also want to check out the records of the 4th Arkansas Infantry (of african decent) from which the 57th USCT was formed. A part of this unit served during the Red River (Camden) Expedition during the time of its reorganization as the 57th. It is possible that Joel was a Officer in this regiment before he was in the 57th.

Anyway good luck hunting.