The men from the 1st Texas Sharpshooters who were at Fort DeRussy did go to Port Hudson when they left the fort. Most of them left Port Hudson before the siege began and went to Mississippi. The two dozen mentioned must have been the ones who were left behind.
If your great grandfather was one of the men who were involved in the capture of "Indianola," he must have been something. I bet your great grandma could have told some stories . . .