The 6 Principles of Persuasion and Influence

Here is a useful summary of some of the basics of persuasion and influence from Cialdini.  Though he states there are 6 universal principles, you might ask if these can be organized differently:  Are there more?  Fewer?  Can these be organized around a universal theory?  What's the evidence for any of these, and how strong is that evidence?  Are some of these principles stronger than others?  How would your rank them in importance? I think he's forgetting THE most important principle: just asking.