1: I would accrue doubloons still and not points. Blacksmith will quickly become too powerful (and especially swing-y in a high player count game); and bear in mind that doubloons normally accrue on cards so that many players can get access to money, especially since the Trading House can become a madhouse.
2: In my only attempts at a 6p game, I found that the plantation tiles actually ran out. You should think about what to do about this and what its consequences are.
Ditto above, plus:
1. you will need more Colonists or a mechanism to move colonists to the bank?
2. More point tokens or trade in points for $?