Wow. Can't believe I never read this thread. That was a great trivia.
Though I have always thought that Oda is using the Momotarou story for the admirals. Momotarou recruited a dog, a monkey and a bird.
But if Oda is also using the seas and its colors as a sign, then Smoker becoming an admiral sounds plausible. Just a random thought though, would he still be Aokiji, or Shiroryu (Shiro is white and Ryu is Dragon right? pls correct me if I'm wrong).
Anyways the mythology about the seas can also be used for the Yonkous, I agree that the black/white difference might reference to Whitebeard replaced by Blackbeard (after all, he did steal the Gura Gura no Mi).
I've seen the design somewhere before, a long time ago when Hancock was first introduced. The fourth guy seemed an awful lot like BrownBeard. Only the 2nd and 3rd guy is a mystery, or they have been used and I just missed it?