The main reasons that backing plates are manufactured and sold for a DA or a rotary specifically is that the threaded arbor on a domestic (sold in USA) rotary is a 5/8" diameter, 11 threads per inch sized male arbor that the backing plate screws onto
. Rotary Flexible Backing Plate, lake country rotary backing plates, makita 9227 backing plate, dewalt 849 backing plate, circular backing plate
The PC style DA has a 5/16" diameter, 24 threads per inch female threaded hole that the backing plate screws into
. Porter Cable 7424 Backing Plates- Dual Action Hook & Loop backing plate for your porter cable 7424, The best backing plates for the Porter Cable 7424
I would suggest choosing a certain size pad that you'll work with most commonly and get a backing plate sized for that specific pad. Such as a 5" backing plate for use with 5.5" pads, or a 6" backing plate for 6.5" pads. If you'll be working with both a rotary and a DA polisher, I'd suggest getting a backing plate for each machine specifically, rather than using an adapter to adapt a rotary backing plate to your DA polisher.
Sure, adapting a rotary backing plate to your DA polisher would work just fine but you'll be wasting a lot of time and you'll certainly have trouble with the thin stamped steel wrenches that come with your DA polisher rounding out and not removing the backing plate as it would be prematurely worn out and begin to just slip instead of working as designed.
If you are going to use the adapter and frequently switch backing plates between your rotary and DA polisher, I'd suggest getting a few extra PC wrenches Porter Cable 7424 Flat Wrench
But instead of spending $4 each on a few extra wrenches why not just buy a backing plate specifically for your DA and just leave it on? You'll be saving time, (time is money) frustration of worn out slipping wrenches and the money to buy the extra wrenches.
As for where to use a specific sized pad, 3" pads with this backing plate 3 Inch Dual Action Flexible Backing Plate, da backing plate, random orbital backing plate
for DA polishers will work well for correcting paint on windshield pillars, side view mirror housings, license plate area and other bumper/grill areas.
Perhaps you'd like a 4" pad better with this backing plate, 3.5" Dual Action Flexible Backing Plate- porter cable 7424XP hook and loop backing plate 3.5 inch.
You can use any sized pads with a rotary polisher, it's pretty much a personal comfort/preference thing. Back in the old days you were pretty well limited to just large sized pads with crude, stiff backing plates. Nowadays we have a much broader choice in pads and backing plates. For instance, here are a few wool pads now available where back in the good old days all there was were 8" and 9" wool pads. Foamed Wool 3.5 inch Buffing/Polishing Pad, spot polish with Lake Country Foamed Wool Pad, wool buffing pad, small wool pad Foamed Wool 4 inch Buffing & Polishing Pad, wool polishing pad, foam wool pad, foam polishing pad, buffing pad, purple wool pad, lake country Foamed Wool 6.5 x 1 Thick Polishing / Buffing Pad Foamed Wool 7.5 inch Polishing/Buffing Pad, purple wool pad, lake country, dual action wool pad Lambswool 6 Inch Leveling Pad (1) Electrified sheepskin wool compounding pad, spot buff wool pad, 4 inch wool buffing pad, lake country wool pad Cyclo 4 Inch Wool Pads, hook and loop wool pads, wool cutting pads 4 inch Double Precision Wool Buffing Pads Cyclo 4 inch Light Cutting Wool Pads The Edge 2000 6 Inch Wool Buffing Pads Buy One, Get One Free!
Just wool pads there other than the traditional jumbo wool pads. We now have a selection of various sized foam pads that would dwarf the above list of wool pads.
I personally prefer 5" pads on a DA polisher and 6.5" pads on my rotary polishers. I find that the 6.5" or 7" pads are a little easier to use than the larger sized 8", 8.5" or 9" pads. Sometimes while using a really large pad with a rotary whether it be wool or foam, you'll be concentrating on one side of the spinning pad (where the work is being done) and you don't realize that the opposite side of the pad had been touching something like a trim piece or another part of a panel with multiple curves and before you realize it you've burned paint with the opposite side of the pad. The smaller sized pads cut some of that risk out of the equation.
Just my thoughts, experience and preferences.