You can if they are different types of ads - e.g. contextual ads from adsense and cpm image ads from adbrite
You can use both, refer this post in google group; http://www.google.com/support/forum/p/AdSense/thread?tid=0cd860836c3cef9a&hl=en
i have used both in same blog. but didnt received any warning. since adbrite earnings are poor, i removed
yes because adbrite is not generated by content rather than by the keywords you enter. however you cant use ypn and adsense together