Pad size and recommendation

Hi,
I'm going to be buying a DeWalt 7"/9" rotary and a friend is giving me a 6" harbor freight dual action. I plan on using the rotary to compound my Boat and possibly polish. I would use the harbor freight for possibly polish on the boat but definitely for wax. I would not use the rotary on my cars but I will definitely use the dual action on paint.
My question is...what size pads to get? Lake country recommends a 7 1/2" for the rotary. What confuses me is what size pads do I look at for each if each is a specific size (ie DeWalt 7"/9" HB 6")? Is there a rule of thumb that if a machine is categorized as a specific size, you look at x size? Or would there be a good size that could be used to interchange between the 2 machines?
Thanks

Sorry, just noticed this is probably in the wrong section. Mods, please move to appropriate section if needed.
Thanks

I would go and get the 3m quick connect pads and adapter, Its worth the money they have 2 different size pads you can get for it I believe they are 9" and 6" I can't remember for sure. For boats they have a wool compounding and polishing pad I would get both. Once you get comfortable with it on your boat I wouldn't be afraid to use it for paint correction on your car you just have to go slow and watch what your doing. Then just use the dual action for a final polish and to apply wax. The only down side with the quick connect is you would have to have two different sets of pads one for each machine.

I'm not looking to invest in a bunch of pads for each machine. That's why I didn't know if there was a good size to interchange between the two. I guess it doesn't matter that much since I really only need a wool pad for the DeWalt since I will be using a diminishing compound (power one totally buff) on the boat. Then I can get a polishing pad for the HB if I want to put a finer polish on the boat. I just thought it would be nice to be able to interchange the pads between the two machines.