Absolutely not. And if my memory serves me correctly you can't even have them on the same site. Many people used to put them into a rotation to get a feel for who was paying better, and I believe Google added a clause in the TOS that now also prohibits that.
Yes, other advertisers can be on your site as long as they aren't formatted the same way as the Google ads. Here is the official information from Google: https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=32849
Yeah we can run bot on same page. Ad it doesnt matter for color or layout of both ads. The thing is google are content based ads and yahoo are not. SO simply inert both of them
The only information that matters is what Google says. If you are displaying Adsense ads and similarly formatted Yahoo ads, they must be in different color schemes.