Well, John D. Rockerduck does make a cameo in chapter four, as the spoiled, bratty son of Howard Rockerduck. I just didn't scan that page.
According to Life and Times of Scrooge McDuck, Flintheart and Rockerduck started off in completely different ways: Flintheart was born poor like Scrooge, and like Scrooge made his fortune on his own, it was just that he didn't share Scrooge's ideals of honesty and "making it square," and had no qualms stealing and cheating his way to the top.
Rockerduck on the other hand was born wealthy, his father being a self-made duck, and he inherited a large fortune -- though there's nothing that implies that old Howard Rockerduck was anywhere near as rich as John D. is portayed in modern stories, so my guess is that the son developed a keen sense of business and was able to increase his fortune to the point of almost rivalling Scrooge.
I think Flintheart and Rockerduck are both interesting characters, both managing to be like a reflection of Scrooge while being wildly different: Flintheart hard-working, tough as nails and not trusting of anyone, Rockerduck as a younger, more future-oriented businessman who delegates and compromises more. Also, while both have had dealings with criminals and don't bother to play fair, Flintheart seems to be the more directly villainous of the two... I can't imagine Rockerduck stooping to attempted murder, for example, while Flintheart has done exactly that on several occasions.
Pity that it's an either/or situation with most writers, the ones who use Rockerduck tend to ignore Flintheart and vice versa. Could have been interesting to see how they related to each other.