The latter is just a rumor someone posted on the Batman section here. I've no idea if its true or not but Tomasi has written other Batman books before so I wouldn't be surprised if he got the call to be honest. As for Tynion staying on Detective until its #1000 there's no guarantee that he will be staying until then however much he might want to. Its still a good year or maybe two away depending on whether it keeps getting the twice a month treatment and Snyder is moving on to the JL so it could be that Tynion follows him onto some other JL related book.