It not that bad, everyone using nikel plated block generally mixing metal since the radiator is brass/copper......
the key is keeping the coolant/water clean (regular clean & change the liquid) and if needed add corrosion inhibitor for fighting the bad effect mixing metal..
or like me (newbiee) just buy premix coolant by the cooling companies..
they pricy, and doesn't have better cooling properties than pure water, but they easy to use and came premixed with anti corrosion & biocide...
edit: (added info) quoted from koolance, the one need to be avoided...
"do not use aluminum with bare (unplated) copper or bare (unplated) brass in the same system. Do not use silver with nickel in the same system."