I think you're not quite following how it works. A page is assigned X ads. The ads are ordered by price. The first ad in the source gets the highest payer (google's assumption is that the first adblock will get the best click through, giving the highest payout for publisher and google). Link units aren't actually ads, they are links to ads, so they can't 'draw away' an ad. They get a separate set.
I don't think anyone outside of Google *knows* it, but it seems like I was told once by some kind of an authority that the highest paying ad will show up in the ad block, even if a link block comes before it. But if you want a guarantee in writing, I can't provide it.