sgaumont,
First of all you must learn your viewable size. Measure diagonally the screen of your 48" center monitor. It is probably 47.6".
Below is your 47.6" monitor entry on PLP Gaming wiki.
http://plp-gaming.wikia.com/wiki/Large_Monitors_By_Viewable_Size#47.6_Inch_1920x1080_.2816:9.29
Under the PLP Layouts section of your entry are listed side monitors that will fit it to line up pixels in PLP.
Note the links under PLP Layouts will point to your setup, but it's in a group of related setups. So you need to scroll its page up a bit to see general details & layout screenshot.
Here are screenshots of the 2 main entries, to give you the idea. You need to decide what will work best for you, based on setup shape & notes listed for each.
Attachment:
Shape040ALT 1920x1080 1280x720 PLP.jpg [ 50.73 KiB | Viewed 1877 times ]
Attachment:
Shape140ALT 1920x1080 1600x900 PLP.jpg [ 60.68 KiB | Viewed 1876 times ]
PS in answer to your questions:
1. These sides listed will not line up with 55" center. That center requires different sides for px lineup.
That said, Windows 10 is reported to allow individual display scaling of multi-monitor setup, which may allow you to reuse sides with 55" (no guarantees, as this Windows 10 feature not yet tested & may look like crap)
2. The sides' resolution is being dropped because there is no 1280x720 nor 1600x900 for purchase, of the physical size you need to line up pixels. But if you find some rare side monitor with the right dimensions & same low resolution, yes it would work the same.