@AvocadoInTheRain:
Nope.
I think Oda is making a lot of this up as he goes along. Famously, Vivi was just going to be a Baroque works officer and it was only on the way to Whisky peak that Oda thought up the whole princess plot, that's why her design looks so villainous when we first see her.
But in terms of the crew, as you said Robin was a last minute addition. Franky was designed with Mr 2's voice actor in mind, so too much of his design can't have been set in stone from the start. Especially since the original shipwright was a small fishman. The plant guy got dropped completely and turned into a powerup for Usopp. Brook's original design involved a cowboy hat, so when that was changed, Yorki had to be invented out of thin air in order to explain who the guy with the cowboy hat was in Crocus' flashback. Jimbei just wasn't present at all in those crew sketches, and his original sketches (along with what Yosaku tells us about him) made him out to be more villainous than he turned out to be. Also I doubt that Oda would have two fishmen on the crew.
[HIDE]https://vignette.wikia.nocookie.net/onepiece/images/b/be/Early_Jinbe.png/revision/latest?cb=20150404100931[/HIDE]
So I think Oda sketched out a few initial ideas but left himself a lot of leeway for crewmates that would strike his fancy later on.
The little fish’s design was later used for a whole underwater village of catfish shipwrights, who aren’t the same biologically as fishmen or mermaids, who all show traits of different species. They’re closer to Pappagu, sea creatures who happen to be sentient.
That’s not to say the catfish’s story was always like that, but then again, we don’t know whether Jinbe was always considered a fishmen himself, if fishmen were originally planned to be called that and actually related to fish, or even if Jinbe was always called that name and related to whale sharks. Like you said, details can be changed, but the main picture has remained the same: Brook was always meant to be a skeleton, Robin was finalized before the story began, and Franky was likewise confirmed to be a large robotic shipwright at that time. I do think Monquito has a point; if we can accept that Oda could have more than one animal-inspired crewmember, then that should apply to both land and sea animals.
Moreover, I simply can’t accept the idea that some version of Jinbe wasn’t a part of Oda’s first sketch, much less that he wouldn’t be in the second one from right before serialization. There’s clear evidence that there were characters erased from both sketches: broken lines, gaps in the lineups, and again, the fact Oda confirmed 11 people were planned from the start. That implies Oda made it clear which two characters would really be joining and needed to be erased. Jinbe was one of them, 100 percent.