I mean she DID change her origin and race when she became bucanneer shaped.
Even still, Nika is dead and is a dead person from CENTURIES ago. We don't even know if he was a giant, just that he fought freely and happily. Her whipping out G3 (when all Luffy had at the start was a pistol)... lmao. So she can turn into a giant but not her bucanneer step-dad?
Blackbeard is also AN EXCEPTION. He is stated to be special and if Oda is REALLY going for the "anyone can have two fruits" that is DOG SHIT. Its a bigger retcon than the Nika fruit.
Bonney having even partial Nika mode is such dog shit.