I assume they pushed Dickey back because of his injury. They just want to give him a few extra days to make sure he's 100% rather than let something linger.
I've always thought Mills is a guy who should be moved to the pen. Decent command (at least in the minors) and a big hook, and the move should tick his velocity up a bit. Oakland is a perfect fit for him though (which I've mentioned in the past).