While I could see Alpha not being a good choice for his poor attitude (and the fact that his name starts with an A like Anakin and Ashoka) I don't understand how you see Rex as a bad addition when he's a clearly convincing character. He's a clone captain with immense loyalty, bravery, and integrity while being a gifted strategist to top it off, which all make him a fine addition to the series, imho. Also, Captain Rex never appeared in the movies because he wasn't conceived until the Clone Wars show. It's also important to note that Appo wasn't seen in the movies until he became commander which was only after order 66, so there's no excuse for Rex to not be Anakin's commanding clone trooper during the events of the show.