Does a pilot follow the PAPI lights in the same way in all categories of aircraft? Since there are different dimensions of aircraft in various approach categories, I wonder what changes pilots in different aircraft have to make when following the PAPI lights. I know that 3° (for example) for one aircraft is the same for all other aircraft, but different categories might have different touchdown points when following the same glideslope due to the MEHT.

Since the PAPI is fixed, what differences are then when flying it in two different aircraft? Does the size of the aircraft change the flight path in any way? Will I always touch down at the same spot?
The lights themselves are fixed, there is no way ATC or the airport operator can simply adjust them to suit the next inbound aircraft. If the pilot needs to they can fly the approach with 3 whites/1 red, but AFAIK that isnt really necessary in most situations (maybe for 747s and other longer aircraft). 

It's worth remembering that the PAPI cannot provide precision to the foot. Having 2 white lights only means you are anywhere between about 2.8 to 3.2 degrees on a standard approach, which can be a considerable range when you're a few miles out.