If I sign up with the Yahoo Search Marketing program, and use it to send traffic to my AdSense sites, is that acceptable according to Google's guidelines? Is it a good idea, do you think?
Would Google like to get traffic to its content network (your site) from the competition and then profit (along with you) from that traffic via Adsense? They definitely would.
Yes, you can advertise with yahoo. I doubt your wallet will allow it or not, since Yahoo is costly place as compare to others.
Do you already use Adwords? Google doesn't mind (in fact they encourage it right on the Adsense site) if you use Adwords to drive traffic and use Adsense to monetize it. Some people think there is something wrong with this but they encourage it. I met a Google sales director and when I posed the question she kinda laughed about those that were concerned about it.