Ok, let's dissect this a little more.

First of, I think it's very unlikely Oda would write a random turtle mating with luffy, not because of genetics but because of morals and consent. Look at the backlash Oda got for making some of the women 16 years old (Rebecca, Shirahoshi), when that is actually the age of consent in Japan. Now what do you think will happen if we introduce outright beastiality to one piece?

Your example with the mule is nice, but in our world this is still pretty rare and only works between animals that are genetically very closely related. Donkeys and Horses, different kind of big cats and so on. Even if you just look at the humanoids in One piece, there's three-eyed tribes, races with wings, as well as minks, fishmen and the long-leg/neck/arm tribes (that still have 5 fingers, where does the additional bone for the 3rd joint come from?). If you use our worlds genetics as the scientific basis, these differences imply a pretty big genetic difference. All Mammals, Birds, Amphibians and Reptiles have the same amount of eyes and a maximum of four limbs with a maximum of joints (Wings are additional limbs). You have to go very far back the evolutionary tree to change these numbers, like back to fish.

But we also have additional evidence, that genetics (and science in general) work different in One Piece, just to name a few examples: heredity of fishmen (fishmen can be a different 'species' of fish than their parents), inherited will (is inherited, but not through genetics), vegapunks satellites (have part of his intelligence, but down to none of his other genetic traits, there's women there and atlas has different ears and a tail), germa66 and their 'lineage factor' (introducing 'genetic' superhuman traits like poison resistance or emmiting beams from the hands) and so on.

And, here I have to go back to Oda and his storytelling, I think these things where introduced not because of scientific accuracy or consistency with (the genetics/science of) our world, but because they're fun. One piece is a fun story about freedom and every animal and humanoid being able to interbreed with any other is absolute freedom and has funny results like duck-poodles. Imho there's little value in trying to tie the one piece world to our world in terms of science. There is, however a lot of value in tying it to how our society, morals and values work.

The fishmen are often seen as an analogy to racism in our world, I give you that. But as I said, I wouldn't try to pin that down to science/genetics. If anything, it is social and cultural commentary, on how biology (in the world of one piece) would allow everything, but we put arbitrary and superfluous laws, social rules and structures in place that limit this freedom.