Yes, Tarkin is in my faction, I just have yet to actually get around to detailing HOW it is that he is there besides having commanded elements of the Ninth Systems Army. With Thrawn, I think that would larger depend on whether or no someone wants to play as the Chiss and introduce him.

As for the Emperor Reborn, I believe much of the cloning shenanigans started well into the reign of the Empire, but that's really a Sigma question.