The reason a DA says 5 in pads only is that the counterweight is pretty closely matched in terms of weight with the backing plate supplied. I have seen alternate counterweights offered online (not from Porter Cable but other vendors). The issue is that if there is a major difference in weight between the counterweight and the backing plate/pad you will get an inherent vibration. Matching these will minimize this effect.
I bought a dedicated Bauer unit for my 3 " DA. I got an extra counterweight and then drilled that to reduce its mass, but it still wasn't close enough to match the smaller backing plate so i added additional mass to the backing plate in the form of additional washers. Works pretty well. Vibration is pretty minimal with this setup. This is still a pretty short throw DA, 6 mm and seems to work fine the backing plate i selected. I really like the smaller pads for the A B & C columns as well as some wheel treatment i do.
If you search around there are some pretty good youtube videos on this topic that deal with this specific topic.
One other comment about the Harbor Freight/Bauer DAs. The Harbor Freight models are no longer available and replaced by the Bauer models. I have not found any issue finding replacement pads for my Bauer DA. I prefer 5 " pads and there are a whole slew of options out there. The backing plate that came with the Bauer (& HF) DA is terrible and i replaced it with a Chemical Guys backing plate. I did check out the lube and both of mine came pretty well lubed but i upgraded the lube anyway. I also have a long throw 15mm DA that i use when the car surface needs a lot more work.
