So you are saying all zoan awakenings follow same rule?
As a reader, all will follow the same criteria...like Logia user body is fluid...with one specific exception(though awakening can be same).
It's never mentioned as rule by oda...but based on the writing/story readers are supposed to get think of the criteria like that...same with Kata/DD who generate stuff and Awakening makes them make surround same as their element/whatever they generate. But we know paramecia has wide variety of stuff...so we can stick to just one criterion...like law and kids fruit...they do follow same thing effecting other stuff but they don't generate stuff so they did not convert anything.
Zoan was said to have increased physical enhancement as we saw in ID and monster point is pretty much a zoan awakening(we all know he is supposed to get a human form but he doesn't have a proper human form at all). So we expect the same.
Oda can either add another exception here for Nika..but the fact remains...like he is not a rubber man..rubber is not his ability is expected to zoan property who had rubber powers so his awakening should be related to human/Nika, not rubber. Or are we supposed to say Nika was a rubber man who had the abilities which are similar to the awakening of DF?
Even that has holes because if Nika is the one who has rubber properties...then he should be rubber only when he is awakened.... not all the time. We will get a lot of issues if we try to think of rules.