I mean, the data points seem to somewhat contradict each other but they do make some sense.
On one hand, there are multiple zoan fruits out there that have multiple models of very similar things. Bison and Giraffe might be rather different for the Ushi Ushi no Mi, but Dog and Dalmatian are incredibly similar. There's no reason that a Longarm, Snake-neck, or Dwarf model of the Human fruit couldn't exist. As far as Fishmen or Minks go, there's no real "average" form for them since they're based on specific animals. Oda could probably cobble something together or arbitrarily pick something if he really wanted one.
On the other hand, Chopper can't actually transform into a full human and there's no real explanation given why. My interpretation is that he doesn't have the Human fruit so much as a generic Humanoid one that gives average humanoid features like bipedalism and humanoid thumbs. Retaining his fur, though, suggests to me that not all humanoids are devoid of fur and thus Minks are more likely some offshoot of people than animals.
But those other offshoots of humanity aren't different species, just mutations of the same species, not even a different genus. I actually looked this stuff up and when you compare a giraffe to a bison, the divergence occurs at the classification level of Family (preceded by the commonly shared Order of Artiodactyla) rather than in the following Genus or Species. If Oda continues to split these differences at the lower levels that makes it less sensible to combine something at the Order or Family level rather than Genus, Species, or even Sub-Species (like homo neanderthalensis vs homo sapiens). If Oda uses this to combine giraffes and bison at the level of Order, then there is no such thing as the Ape-Ape Fruit or Monkey-Monkey Fruit because they share the same Order (Primate), and Family (Hominidae), as us, so by not dividing things up more finely, Oda's lumping all variations of humans with all variations of every great ape and monkey which I would be willing to bet very few people would be completely comfortable with. So if we find an inconsistency with this pattern just for the sake of…whatever Oda's whim is, then there could probably be exceptions but we'll have to wait and see if there has been any kind of primate DF other than Hito-Hito no Mi and if it's called something different.
Someone mentioned Smiley having gills. Isn't the species of his DF amphibious? Like, is it structured in such a way that it can breathe underwater indefinitely like a fish or are there limits on how long it can do that, like aquatic mammals?