I have a couple of theories about how Fred & George managed the Rita Skeeter prank. We know from Quirrel's response that Rita Skeeter's article included evidence from a variety of sources, difficult to fake, and certainly costing more than 40 galleons. The obvious implication is that either they got some covert support (Professor Quirrell/Tom Riddle?), or they found a simpler method. I can think of two.

1. Modify Rita Skeeter's memories. Make her remember viewing all the evidence, when in fact she hadn't seen them. Hiring someone to do that probably wouldn't have been all that difficult.

2. Impersonate Rita Skeeter with Polyjuice and turn in the completed article to her editor. Maybe some memory modification as well, to make her not deny it.

This passage does perhaps contain a subtle indication that this is the case:

"How very foolish," the man said dryly. "It would have been wise to memorize the face of the disguised Death Eater training Harry Potter to be the next Dark Lord. After all," a thin smile, "that certainly sounds like someone you wouldn't want to run into on the street, especially after doing a hatchet job on him in the newspaper."

Rita took a moment to place the reference. This was Quirinus Quirrell?

I would expect that if I wrote a newspaper article, even if I didn't memorize the face of the subject of my article, but I would expect that I would have seen it at some point. Rita Skeeter, however, seems completely ignorant of what Quirrell looks like. It is possible, however, that this would be the case if the memory of writing the article had been implanted.

This seems extremely subtle, even by EY's standards, and there are some alternate explanations I'm considering as well. However, I still think that these methods are much more plausible than any theory of Imperiusing centaurs.