Oh gosh… Jinbei's width is too great, he will have extreme difficulty in fitting through doors, and Franky fixing those doors will be a daunting task, not because of the difficulty, but because of the panel appearances to have that installment.
Jinbei was a character, from a design point of view, made to NEVER fit on the dream ship of the future Pirate King, meaning that he was never made to join.
That quote about him is not coming true, and babysitting is not a reason to join the crew... Ace isn't confirmed dead, all it says is that Luffy will be moved to an escape vessel, then Ace will be reborn.
Amazing, Jinbei is kind to fish, and can talk to them!! Just because he is the only fishman that has been shown to do that, it doesn't mean that he is the only fishman in the world to do that... He's obviously not bound to be since FI can have the same character like him, but without width issues. Furthermore, that character won't be SUPER STRONG since Jinbei's power can be in question.
I do agree that a fishman will be the helmsman, however, I don't agree to Jinbei being that Helmsman... I feel that Oda will probably introduce a new, more appeasing fishman to join the crew and leave Jinbei as an ally like Hancock and all these other characters... Crocodile, Bentham, Buggy, Galdino, etc.
That being said, Perona's situation is very different. She's in a far more favorable position than Jinbei, and her position entitles her to follow a person into the future, where she's able to secure the spot.
That is an important reason why Perona would join over some other character... Which happens to have a revealed identity.
Jinbei, a plot device, is nothing different than Boa Hancock, except he has a future island to attend to, which the SH crew plans to visit this present time.
When FI does come, and Jinbei is there, then that character can be considered to have a chance to join... However, chances are that Oda may create a new character to take the slot of the Helmsman, for whatever reason that may be... Furthermore, this character can appear in the future, where an obstacle concerning steering appears and the crew cannot properly handle it without a skilled expert.
That may also happen, but I personally do not think that Jinbei is going to join at this moment, he needs a lot of things cleared up, and with the previous chapter, in my humble opinion, his chances have literally started to drop, even worse than his width issue...