I've always imagined that Melo was heavier than he actually is easy. He uses his build well, I think he would be amazing at the 4 here.
I doubt Ariza is not a starter in the nba anymore. Much like Battier at the same point in his career. The rockets need to keep players who don't need to be guarded off the court. They start Ariza and his man is the help guy. I can see when they go small with either Gordon or Tucker, carmelo playing the 4. Personally, I think the hold up in the trade is Ariza and Capella.