Shisui was never in Anbu to start with. He was just in the Police Force. He was 18 or older. Lol
Kakashi would be the captain, since he's more experienced.
Itachi could be the little prodigy of the group and was the best at genjutsu. (He's the best genjutsu user. Shisui just got hype for being the only MS user when he was alive, but even so he could only use each Koto tech once a decade. That's pathetic. Itachi was better than him even at time but didn't receive the praise since he completed missions primarily for Anbu- so in the shadows. Shisui was completing missions and feats in front of people so they knew what he was capable of unlike Itachi. Regardless of this, in Sasuke's part one flashbacks Itachi was still stated to be the most elite of the clan despite being younger and only 13, while Shisui was still alive. Lol )
I'd rather see Yamato as Kakashi's and Itachi's support. He'd be more helpful. Shisui was good for his day, but that generation of Uchiha was being surpassed by a 13 yr old Itachi. So, it doesn't say much for Shisui's regard. :/