I have some pieces of brass in my layout, but I got them from another layout I purchased at an estate sale. Like has been stated above, brass oxidizes faster, but both are copper mixed with either zinc (brass) or nickle (nickle silver). I have not seen brass at the LHS, and last time I talked to the, they said they last sold it 8 years ago and recommended I don't use it as the nickle silver works so much better, less maintenence, etc.
You can safely intemix them as you need to.
I run my railroad like the prototype....IN THE RED!!!!!!!