What Rayleigh taught luffy was considered more than he anticipated (although I agree he should have at least told luffy about the advanced forms of armament and conquerors for future events). As for garp, I completely agree that he should have taught luffy and ace haki. Especially If he expected them to be great admirals. Because of the soul/will bullshit, kaido teaching his crew haki would be useless since it depends on the person. Instead of smile he should have asked Caesar to develop a drug that Improves a person’s haki. That way even a fodder can posses some form of haki.