As long as they qualify and complete the Federation's equivalent to the command officer course, I see no problems with them being a CO or an XO. I do have problems with one being a CEO then becoming an XO within a week's time. If I was serving under that person, I would go for a transfer because I doubt their abilities to command. Just because the Captain says your the XO doesn't mean you should be the XO. When I read that in Rough Beasts of the Empire about Sisko, I just rolled my eyes, unless Sisko left the ship to go through the course.