If you buy an expansion like Knights and Cities or Seafarers then you also have to buy the 5-6 expansion for that particular game. We enjoyed Knights and Cities, but I think it would be a zoo with 6. Actually, I don't think I would ever try Catan with 6, seems like it'd really drag.
6 is sort of a lousy number for board games, IMO. Fortunately we don't have that number too often.
The rules change a bit with 5-6 players because anyone can build after the player whose turn it is has built. The game does seem to move faster with this rule.
If you have someone unfamiliar with Catan, I would just go play without the other expansions first.