@BinSquarr:
++ Was gonna' say below Smoker, so Captain maximum, however, Purin-Purin was a commodore, a he didn't last a chapter really. So Commodore. Sanji, Zoro, Robin at the same level I'd say.
I don't see Robin having a very long career in the Navy; between her ability to read Poneglyphes, her intelligence, and her Devil Fruit ability, I think that Spandain or Spandam would eventually go over Sengoku's head to get her into Cipher Pol. If not, then she winds up in the Naval Science Department or Naval Intelligence. Where Robin winds up depends on which organization wins the fight over her.
Zoro probably has the highest chances of advancement within the Navy of any of the Straw Hats, especially if he gets into a role that he's well suited for such as bodyguard for a high-ranking officer or government official. While Sanji is close to him to in strength, sexual harassment is going to be an issue for him.
Nami, Usopp at Ensign.
I'd figure on both being quite a bit higher actually. Nami's natural navigation abilities would be in high demand within the Navy as she's one of the few people who can feel what the Grand Line is up to.
Usopp, on the other hand, probably doesn't have the same abandonment issues with Yasopp if he goes into the Navy, so I'd figure on him being much, much powerful than the regular Usopp that we see in the series. Rather than weakness, his real problem is lack of confidence in himself and his abilities.
Franky probably not a marine guy probably be a Pacifista…
Chopper's a reindeer, wouldn't have him in my navy.
Vegapunk works for the Navy, but that does seem to be good destination for Franky. He'd either be an Naval attache or a member of the Science Squad.
And as far as Chopper goes, I don't think they'll have trouble with that sort of thing. Look at the people Cipher Pol employs after all; if the World Government is willing to let Wanze work them, they'll let in anybody.