However to address the first point, many of the reference books I have seen in regards to Sharps bullet sizes vary greatly. In fact, really on the .58minnie has as many. Not counting the slant breech models ranging in the .44 - .45 calibers.
I have seen both the tie ring base, flat base, and hollow base sharps range from .520 , .535 , .538 , .554 etc. Many of these are dropped as I would think after being fired, the combustion would increase the diameter of the bullet.
There were also Confederate Manufactured Sharps cartridges, which leads me to believe that they would obtain cartridges when possible. However, to throw another wrench in it, we have found (personally found) and seen other examples of field cast sharps projectiles. What they did with these I am not sure. In Thomas Stelma's book, he even references imported Sharps cartridges that are identified as Confederate.
Here in Dover, we have hunted some of Forrest's areas of actions and camps. We have found Maynards, Colt Revolving Rifles, and Sharps. It was my thought that they were carrying M1859 Sharps and they possibly were some of the weapons he had obtained from Kentucky.
However, that late in the war, your mentioning of the captured weapons from his 1864 Campaign is more highly likely.
John Walsh