I've always struggled to accept the idea that Duncan was a power forward but Garnett clearly was a power forward for most of his career. In fact he played far more minutes at the small forward position the first fifteen years of his career than he did center.
I'm glad you mentioned KG as a SF here because that's how I viewed him most of his run in Minnesota. It wasn't until he went to Boston that he truly became a fulltime PF, in my opinion.
He was LeBron like in that he was moved all over the floor in Minnesota because of his skills and the talent they surrounded him with. I know there wasn't a lot of talent there, it was mainly they'd take what they could get and KG had to make it work.
His MVP year when the Wolves made the WCF though he was primarily a PF and started to really assert himself as an all time great.