Airlines: Europe plays well with 6. We've played it a few times with the 6-player rules and found that the game doesn't take any longer. With 6, each player generally gets fewer turns and fewer decisions which can
lead to extra AP. Fun, none the less.
Acquire on the other hand can be a nightmare with 6. One or two players always whine because they get left out of every merger and spend the bulk of the game just adding tiles making the other players rich.
When we play 6-player Acquire we make it a team game with players sitting opposite each other adding their scores together to get a team score.