Is a mix of copper and galvanized pipe a problem if dielectric unions are used where they meet?
Dielectric unions reduce the rate of galvanic corrosion that occurs when dissimilar metals are in electrical contact.  They don't eliminate it but they slow it down.  How much does occur also depends on the mineral content of your water.
You should always use a dielectric union when there is a transition between one metal and other.
Dielectric unions are not needed between metal and any type of plastic pipe.