IMO, Picard was one of the weaker parts of the original show. At the time, I wanted him to stay a Borg and have Capt. Riker, Shelby and Data as the leads. I would trade him for more Shatner/TOS, ten times out of ten, with no hesitation. YMMV of course.
The Excelsior was designed to be the "ugly villain ship" of ST3. I know a more filmable model would need to be made, but the initial draw of this thought excercise, for me personally, was imagining an entire series of wonderful angles and shots of the "A" as a weekly special effect.
As the older TOS regulars are slowly replaced (McCoy with Pulaski, Scotty with Geordi) and Spock only as a recurring, anyways, you could definitely get away with the TOS Admirals bit, but I don't imagine Kirk stepping down any time soon when he JUST got his command back. Even in the official canon, there could be a 3rd 5ym there.
When Shatner is finally ready to move on, you can introduce Picard, as the new Jellico-style hated incoming Captain. LMAO!