They make a 5-6 person expansion specific to traders and barbarians. You would also need to have the original Catan game, the Traders and Barbarians expansion, the Traders and Barabian 5-6 player expansion, and regular Catan 5-6 expansion.
don't see why it would not be. the pieces are all made to fit standard catan hexes and replacement hexes are std sizes - for example, the fisherman tile replaces desert tile. this expansion has great replay value and is very versatile - it combines several smaller expansions and you can mix and match. the fisherman expansion is by far my favorite. I still have not tried the barbarian variation yet. basic catan rules are straightforward and these variations just build off it. for example, there is a card deck with events that replaces dice rolls.