Yes, purchase of Crimean surplus would have been a useful addition to the Confederacy, as well as the British East Indian merchant fleet laying idle in many British ports.

A useful site is located at http://www.pdavis.nl/Russia3.htm , this regarding the use of the vessels you are considering at Sebastopol and Kinburn. Mention of the three French armoured batteries used in the attack on Kinburn. They were the earliest armoured steam vessels used operationally, to my knowledge.

You mention the usefulness of shallow draught vessels in waters of Florida and Texas. The iron hull steam vessels made on Grave's design by Thorneycroft on Thames at the end of it all were an effort to remedey the situation.