Another thing detracting credibility from your theory is that not only is the head-shape thing not shown to be hereditary in-universe (Candace and Isabella both have semicircular heads, and they're clearly not related), but Phineas Flynn and Heinz Doofenshmirtz also have different variants of triangular head shape. Phineas, for example, has a long pointy nose, while Heinz has his chin as the point of his triangular skull. And this can't be chalked up to art style, either; Heinz and his father both have the chin as a vertex, whereas Phineas is always shown with the nose as a vertex. Even Marty the Rabbit Boy, the show's other triangular-headed character, has a different facial structure; Marty's vertex is simply the edge of his face. With such wild anatomical differences, it's a miracle Phineas and Heinz are even the same species (actually, legally speaking, they're not!).
I'd also argue that though Phineas and Heinz both seem to be master engineers, that's maybe the only quality that the two share. Heinz is a classic screwball with a fractured childhood, unconventional relationships with others, and deep, unresolved animosity. Phineas is a curious, inventive, optimistic boy for whom life seems to be a world of pleasure - he's rarely ever seriously upset at anything (outside of special episodes, as Buford remarks).