There has been much discussion of chemical biocides on this and other boards, including sulfates, boric acid, hydrogen peroxide, and others. ALL have nasty properties that make them undesireable in a loop, including human toxicity, nasty reactions with the metals in the loop or low effectiveness. (I forget the details, but I'm sure a search would find most of them.
If sanitizing a loop were an easy thing, we'd all be using these chemicals, and this sort of endless discussion wouldn't be happening
As far as the difference between Cu and Brass is concerned, there is relatively little problem. AFAIK, ALL modern cores (and most older ones) use a mix of Cu and Brass (plus solder) because of the difficulty of making radiator tanks out of pure copper. The alloys involved however are such that corrosion is not a problem between them, and anything that will attack one will also attack the other - bear in mind that brass is mostly copper.
Gooserider