noob question alert! Are all characters in the main story dropable?

Like can I get characters like arlong, crocodile... from beating them over and over until they drop?

Yes, this units can drop in the story, so you can try to get them there (but there drop chance is really low, some people get them after 5 times, other dont have them after 200 times...)

So Crocodile is the best farmable character from the stroy, he is really worth
Also Mr 2 is really good if you use him with Garp, you can get Mr 2 also from Rare Recuit, but yeah....

Arlong is also able to drop and he is a really good unit, BUT right now they have started again with the Fortnights, so in about 5/6 weeks (I think so...) will the fortnight from Arlong be back
It should be much easier to get him there, instead of farming him in the story