1) I think google likes being the only ads on pages, they dont like you mixing. (not 100% on this, i dont use adsense)
2) Theres tonnes i made a list here:
http://www.websitebabble.com/general...rtunities.html
My top ones are: Adengage & Bidvertiser
Alot of the other sites are great but i dont meet thier traffic requirements (most of these are CPM though)