Dan,
I too had the same problem with Buddy and Kailee when Kailee first moved in. Of course I was having fits because Buddy could only have the lamb and rice, and I was gradually changing Kailee over to it, so I really didn't want Buddy into hers. They did eventually work it out, and the novelty wore off. After the first couple of weeks they stuck to their own bowls. However, Buddy was used to being able to leave some of his food, and go back and nibble later. This ended forever after Kailee came, because she always cleaned out what was left in his dish. I say give it time and the fun will be overwith.
As for changing Maggies food, it should be fine. Buddy was off of the Puppy formula and onto adult by 7 months. The puppy formula had too much protein in it and was causing him some problems. He did fine on the adult formula.
Just sounds like when we were kids growing up, one of my siblings always had something on their plate that I wanted. Just give it some time