Although I personally don't see Jimbei joining (im a Hancock supporter but thats partly because I am riding on my prediction I made during the amazon Lily arc and im 4/4 so far on my predictions with 2 predictions left to be decided :P I have nothing against those that think otherwise or want Jimbei to join) But this statement is just what im getting at. We know NOW that Zoro and Robin are small fry in the larger picture of the world 'rookies' as they are called. But we were not meant to think that way when they showed up in the story. What could be rediculous over the top character now might be nothing more than a small fry in the bigger picture that we just havn't grasped yet.
Don't put strength and notoriety in discussion as a means to disqualify someone from joining as there is precedent within the story, or at least within the context of the joining, of really strong characters becoming strawhats. Or at least they were strong considering the scale at which they were revealed. Zoro was infamous to the point where every pirate on East Blue knew his name and even feared it, including Buggy and Arlong (although arlong didn't neccesarily fear it, but was aware of him). Robin was the right hand of a shichibukai itself which at that time was an unheard of person of strength until we understood who she was and how her powers worked.
The thought that everyone on the strawhats joins as an underdog is simply not true, its just eventually they must face someone stronger than them to make the story consistently full of danger and excitement. I bet if Oda decides to let either/both Hancock or Jimbei to join, he will have plenty of things in the new world to challenge them (By the way I don't honestly believe that for Jimbei, im just supporting you because it supports my statement, he knows too much of the new world for this part of the statement to qualify XD).
We know nothing of the dangers of the new world, we cannot honestly say that all the shichibukai can be placed on such a godly tier that there is no way one could ever join the strawhats. Their powers were actually minimized in this very arc of how important they are in the grand scheme of things. They are merely sanctioned pirates who are called to assist the marines in a time of emergency and as sanctioned pirates their name simply gets around really easily.
I'm guessing that you quoted me to support your statement instead of to argue with me, based on the things you're saying. I will say that I was pretty high on Hancock when Luffy first got to ID. Once I saw Jinbei, I simply decided that he was a better fit because he was a fishman and not a fruit user (though I still like Hancock).
I want to touch on the statement about Jinbei knowing the New World. We really don't know how well he knows the New World or how deeply into it he's traveled. My guess is that his knowledge would suppliment the New World in the same way that Robin's knowledge supplimented the first half of the Grand Line.