I think Grant Williams can be part of the players who are the exception to the "rule" of having to be properly sized for a position. He has good qualities to play PF: he's very strong, has good length for his height, has decent speed and is a high IQ player who has the mentality to "play big". The latter imo is the most important, a player having the right mentality for the position will likely be able to apply their strengths well and minimise their weaknesses, such as boxing out, knowing how to play help defense as a post player, making the right decision to roll/pop, crashing the glass, etc. To me Tatum and Hayward lack the necessary mindset to play PF, which is why I'm not in favour of starting Brown/Tatum/Hayward in the forward slots. I'd rather have Hayward be the sixth man and only play small when needed.