I moved to a house that is over a century old and noticed most of the upper
floor water lines to be
newer copper but the lower main lines galvanized steel

Won't this lead to rapid deterioration of the lower water lines?

I would also like to know general guidelines for avoiding galvanic action on
piping and so forth
I need to have a water heater installed andwas told to use fittings which
use a
mix of different metals in it's construction...
Why is this, and how does this negate galvanic action by merely being a
mixed metal fitting?

Thanks
Joe