You do not need isolation fittings like a dielectric unions or nipples when mixing ferrous and non ferrous metals like copper to black pipe ESPECIALLY in a closed system where little or no oxygen is present.
Think about the following
Steam System Black steel with cast Iron radiators and BRASS valves.
Fire suppression piping BLACK STEEL Brass OS&Y valves and BRASS sprinkler heads.
Gas piping Black steel Brass gas valves.
Water Mains Cast Iron class 22 with copper Type K gooseneck connections
Fire hydrants cast Iron Brass nipples for the fire hose connection.
Depending on the type of service and piping materials like copper/brass, etc., connected to a galvanized line here could be a major problem.
Closed hot water or chilled water system not a problem as long as your not mixing zinc.
This was a major problem in by gone days when folks trying to conserve copper made an alloy called yellow brass in stead of red brass.
The red Brass 85% copper 15% Zinc gave years and years of trouble free service BUT the yellow brass 60% copper 40% Zinc had all kinds of problems like becoming brittle like due to dezinification
Using a dielectric fitting is not a bad idea but in this case it really is not warranted UNLESS you have a major make up problem where fresh water (lots of oxygen) is in this system as Oxygen in water is what destroys ferrous metals under most conditions.
Even the older cast Iron water mains are protected by use of hot applied Coal tar enamel.
The better codes (not the codes where officals who are being paid off by some home builders) do want some type of protection where a conditions may exist where a premature piping failure may occur do to mixing various materials.