To the OP:
Since you said you already purchased
Meg's 6.5" inch pad, you have to take some things into consideration.
Meguiar's pads do not have a full-back velcro like many LC pads. Because of this, even if you wanted to use a 6" dia. bp with 6.5" pads, you cannot do this with Meg's pads as your bp will be larger than the velcro.
Along with LC pads in various sizes, I also have 6.5" Meg pads and even with Meg's own 5" backing plate, what ends up happening is the backing plate cuts into the foam pad when you apply pressure. Part of the problem of the Meg's backing plate also is that it's not flexible (it's a hard plastic) and it has a sharp edge compared to the lake country backing plates, which are flexible and has a rounded edge.
I don't have any experience with 5.5" pads, but from the feedback on here and from experience with 6.5" pads on a 5" bp, 5.5" pads sounds like the way to go on a PC. FWIW, I much prefer my LC pads and backing plate vs. Meg's.