Melo should be the PF for exactly the reason Briggs mentioned. It's a scoring position and that's what Melo does best, and he's got the size and rebounding to do it.
The issue with most of these defensive centers is that they're ultimately role players. Protecting the rim DOES have value in the NBA, and teams need that to win a title, but great rim protecting centers tend to be marginal at other aspects of the game. Roy Hibbert is practically the best rim-protecting center in the NBA still, but he doesn't do much else, so when Indiana doesn't have a lot of good players, they're still bad. Mozgov is a good rim protector that helps Cleveland win, but you can overpay for him when you have 3 real difference-makers. Teams like the Knicks have to spend resources on difference-makers first, then add the rim protector when they're ready to make a run.
The only good rim protector in the league that you can actually run an offense through is DeMarcus Cousins...