Tarkin is a governor. Someone can have different powers and authority without outranking him.
Well Tarkin was a military governor therefore should be above generals/admirals. I just hope they won’t crap on Tarkin’s character. The worst case scenario would be to make Tarkin merely a construction overseer, while making the new guy the initial first choice for Death Star command and then when he will be killed during the Rouge One they would establish that Tarkin was just a replacement (picking up the scraps).