I'd agree that there are various types of Haki, since so far we've witnessed quite a few abilities that could be explained by that

Vision Haki AKA Mantra - We've first experienced this in the Skypea arc, and if this turns out to be Haki then Oda is my fucking hero

Strengthening Haki - The type seen in the Amazon's arrows and the fat snake's staff

Defense Haki - Could be just the above mentioned haki only applied to specific parts of you own body, or something else entirely since we've only seen a few people do it. Would also explain why the sumo wrestler from back then claimed to have the best defense in the world, perhaps he possesses the strongest defense haki in the world. Could also have something to do with CP9's Tekkai

King's disposition - From what i can tell, the only Haki that can be used as a weapon all by itself. Can't tell if it actually does any physical damage but just knocking people down looks pretty awesome by itself too.