The case you are describing, that you address people with whom you are on a "Du" basis and only have others which are not in that category under CC, then it's perfectly valid to address the main recipents as "Du".
It gets more complicated, if you have some main recipents on a "Du" basis, and some not. I constantly have this in my projects, where I usually am quickly "per du" with the engineers, but not with the managers. And if I need to communicate to both, it's not easy. In the headline, I can use something like
Hallo Herr Müller, hallo Klaus, ...
But within the text I usually try to avoid the direct addressing or - in case you are in closer contact with them - use "Ihr", allthough this belongs to the "Du" category.
At a side note, it also depends on the topic. For example in our company we are all "per du" even with the CEO. But if I get an official information from him, for example concerning my contract, I am always addressed as
Sehr geehrter Herr...
But always when having to deal with such problems - I constantly wish we hadn't that damn thing in our language... ;-)