Wish I could re-find the article which was a pretty comprehensive piece on who makes up the mercenary forces in Iraq.
But after a quick search one can find thousands of articles with references about Blackwater and Kurdish fighters (Peshmergas) such as
http://www.tagesspiegel.de/zeitung/Sonderthemen;art893,2376903The insurgents are mostly Arabs, and the company Pirate and Steeler work for believes Kurds are less likely to be infiltrated. They also have a long tradition of fighting against heavy odds. . . Not all private security contractors are foreign run; Pirate and Steeler work for a Kurdish peshmerga (literally, "facing death") commander. Most of his employees are Iraqi Kurds, but he also employs a dozen former Lebanese militiamen, ten Americans, and one Canadian.
http://globalresearch.ca/index.php?context=viewArticle&code=MEY20060518&articleId=2461According to Global Security, “Erinys Iraq Ltd is the private security company hired to protect Iraq’s oil pipelines under a US$40 million contract awarded in August 2003. Erinys Iraq is an affiliate of Erinys International formed in 2001, landed the Iraq contract to supply and train 6,500 armed guards charged with protecting 140 Iraqi oil wells, 7,000 kilometers of pipelines and refineries, as well as power plants and the water supply for the Iraqi Ministry of Oil. A majority of Erinys’ workforce (15,000 Iraqi and 350 international staff) in Iraq are Kurdish peshmerga.”
http://www.investorsiraq.com/showthread.php?t=53546Mr. Veldwijk said his firm hires 150 to 190 peshmerga, Kurdish fighters loyal to Iraqi Kurdish leaders, to protect its two exploration blocks. "It's not cheap," Mr. Veldwijk said. "It's not so much the peshmerga who make it expensive, but the interface on the managerial side to make sure it works well."
According to P. W. Singer, the author of Corporate Warriors: The Rise of the Privatized Military Industry, their pay ranges from $250 a month for Kurdish fighters to $1,000 a day for former Green Berets.
So from these you can kind of piece together that we hire Kurds because they won't leave for better pay to fight with al Sadr.
Sorry I couldn't find the other article, it was a great summarization of the makeup of security forces.