I have both standard Catan and Catan Seafarer, thinking about buying the player extension to expand both games to 5-6 players. If I only buy the Seafarer's player extension, can I also use it onto my standard Catan to play with 5-6 players? Or do I have to buy the 5-6 player extension for each game?
You need both player extensions in order to play with 5-6 players on Seafarers. The Seafarers 5-6 player extension doesn't contain the pieces needed for the extra players. You'll need the frame pieces, extra titles, houses, roads and cities from the regular Catan 5-6 player extension in order to play Seafarers with 5-6 people.