I know for example that you can't have YPN and Adsense on the same pages. Can you guys point out other companies that are acceptable to use on sites already using YPN? Thanks! (My apologies of this has been asked many times, couldn't find it in a search)
You should be able to use in-text advertisers like Kontera. The basic rule is that ads can not be confused with YPN or AdSense.
Anything that is not contextual. Amazon and Ebay would be fine, I believe. I would not suggest Amazon's books though as the pay is lousy.