Usopp's dream is to become a brave warrior of the seas and to become the real sniper king. It's an additional dream he took on just before the timeskip.
And when I mentioned their backgrounds I said I was only referring to everyone that joined in the grand line onwards, so Usopp's not included in that.
Did Ussop actually claim that as a dream? I don't recall one way or the other. You say the pattern only starts on the grand line, but I would argue it was there in east blue. Sanji is the chef, his dream is to find all blue, which is considered a chef's paradise, his fighting style is all kicking because he needs his hands in good condition to cook, plus his moves are all named after cooking terms, and he comes from Baratie, a restaurant of fighting chefs. Even Zoro fits. He's the swordsman, his dream is to be the greatest swordsman, obviously he fights with swords, and he grew up in a sword based dojo. The only exceptions to the pattern so far are Ussop, at least until the sniper king dream came about and since he does not come from an island of snipers, and Nami, at least before she got the clima tact and since she does not come from a place filled with cartographers. It's important to note that not all of the crew members fit this pattern when they joined, so while Carrot gets bonus points for already fitting, it doesn't disqualify Yamato either.
Edit: Now that I think about it Chopper and Robin's abilities have nothing to do with their positions or dreams. There are actually a decent number of exceptions, but the pattern generally holds up.