Everyone has a different opinion, but mine is that your Rottie will be JUST fine (probably better off). Larger breeds have the problem of getting too much protein with regular puppy food, which makes their bones grow too fast. We switched our Golden pup over to adult food at just about 5 months. The national company that raises seeing eye dogs does the same (at about 5-6 months). There are some that feed puppy food for the whole first year, but I'm not a believer, and a lot of other people aren't either. Regardless, you're a-ok =)