The comma to which you are referring in rule 1 is called the Oxford comma. There is hot debate about whether or not to use it. In my experience, the Oxford comma reduces the chance of confusion and certainly doesn’t cause any harm. Omitting it, however, can result in a misunderstood sentence. Here is an example that omits the Oxford comma, and thus may create confusion:

“In the invitations, Tom invited strippers, Clinton and Trump to the party.” It may suggest that Clinton and Trump are the strippers. Compare it with: “Tom invited strippers, Clinton, and Trump to the party.”