You can get work done the way the HF da is. I have one the backing plate is a little big yes, which causes the HF pads to start to shred a little after using them for a little bit. You can cut the HF backing plate to lessen the diameter, it is pretty hard material. Smaller backing plates can be ordered from many places, even auto parts stores may have them, may not have them in stock but could get it for you later that day or the next day from their warehouse,
You don't have to change the grease out, the DA is fine, you won't be running it for 7 hours a day trying to make a paycheck out of it. My suggestion is to buy it with a few of each HF pad that you think you might need and go to work with it. Like I a said after using it for a while the pads might start to throw foam from the backing plate being larger than the pad back Velcro. But then again you might be using a polish or compound that is very dusty anyways