What about coastal ironclads based on either the Confederate or Union models?
That's a great question. Even though they make sense, I decided not to use coastal ironclads for a few reasons:
1. Between 1856 and 1875, the biggest threat to Texas is on her frontier, not at sea. Most of the defense budget (up to 80%) goes to the Army, who is fighting plains indians. Conditions are worse during the American Civil War because all the US forces that had been on the US frontier are now back east. Plains indians are unchecked and take advantage of the gap in coverage.
2. Defense is consuming over half the national budget, which amounts to 10-20% of the GDP. When you spend that much of your GDP on national defense, other programs suffer. The navy takes a backseat until the Comanches are finally run out of Texas in 1874.
3. The navy has an authorized strength of only 700 men, so I felt the money would be better spent on seagoing vessels. The Naval end strength won't increase until 1881, when other Latin and South American navies start expanding, and Texas begins to focus on her vulnerability at sea.
That was thought provoking. Thanks for the question.