Re: Billing order.
Shawn and Nicholas aren't heavy hitters as actors or characters so it doesn't surprise me they didn't make the main credits.
The ones that did are the ones you tend to think of first when OT/FC/DOFP pop up in your mind. They are the iconic characters and the most famous actors.
As for the billing I'd say Halle/Anna were equally billed at #5. Ellen/Peter at #6 and Ian/Patrick sharing the 'And' honors. Hugh, James, Michael, Jennifer were 1, 2, 3, 4 billing - to be expected. This is how I thought it would go. The credited cast are the heavy hitters of the series.