The only different between McGinn and Downie and what you're considering a power forward, is their weight. They play the same style. Their job is to go hard into the corners and get the puck to Duchene, finish their checks, be hard to play against, and go to the net for rebounds, screens, and passes. This is Iginla's style as well.
Their weight has nothing to do with how well they'll click with Duchene. Comparing Stewart not working to Iggy is a bit of a slap in the face too. He doesn't have the work ethic or smarts that Iginla has.
That leaves just one power forward that didn't quite work in Landy, and he's young himself. Him not being able to adjust quite as well may have been due to his own inexperience, the same way Duchene found it much harder to adjust early in his career.
I don't see Duchene's past as a reason to doubt how well Iggy may work with him. Iginla's one of the most skilled power forwards in the league, and doesn't need the puck on his stick to be successful. That's a perfect fit for Duchene and O'Reilly who are great with the puck.
He had an issue finding chemistry earlier in his career, but since then he has found chemistry with many players. Mueller, Fleishchman, PAP, Downie, McGinn, O'Reilly, all guys he clicked very well with. The first three were more his natural fit, while I doubt he would have clicked with the latter three when he first broke into the league. It was all part of the learning curve he had to go through coming out of junior, and why he talks so often about how much he learned since then.
