I’m not creating anything, I’m trying to make sense out of this. I wanted him to have an Oni fruit, actually.
Don’t forget that his fruit is mythical fish, not a regular one. So don’t expect his full Zoan form to work like a normal fish, whether it’s a dragon or not.
My problem with your theory is that you’re classifying dragon as fish. Even if we talk about a sea dragon, I’m not sure it can be considered a fish, especially a dragon that has legs.