Post extensions allow you to expand a post-bound album. They usually come with the packages of page protector refills. You can also use the Pioneer "Snap Load" system. It takes the place of the posts. I prefer using a D-ring album. It just holds more and is easier to work with.
As far as my albums go, I have many with different layouts in them. For albums with just one theme, though, I've learned to plan my pages ahead of time to fit within the album I've chosen.