The only thing(s) that Pumpkinhead and the A:R Aliens have in common are the teeth, the avian legs (they don't look alike too, it's the anatomicals structure that bears resemblance) and the humanoid ribcage. End.
The comparison with Ripley #6 especially I don't get - the thing got serried teeth, in a very little mouth. Pumpkinhead has a big, big mouth, with humanoid teeth. Ripley #6 has little, bloody eyes on the side of the skull. Pumpkinhead has them in the front of it - and they are white, pupil less. Also the clone hasn't a head as detailed.
Also it wasn't ADI designing Pumpkinhead if I recall right, t'was Winston.
