Galvanic Corrosion

.....and what's wrong with using cast iron valves instead in the larger sizes and cast stainless steel in the smaller ones?

Alternatively, run the smaller size pipework in copper with bronze valves, or even an entirely plastic system, perhaps?

This site explains it very well:

Galvanic and Corrosion Compatibility Dissimilar Metal Corrosion ...

The reason that joining two metals together may cause accelerated corrosion is that some metals are more easily corroded than others. This goes back to basic chemistry. All engineering metals react with their environment, and some are more reactive than others. The rate of reaction depends on the environment; when dissimilar metals are electrically connected to each other, each interacts with the environment, and the more reactive metal takes the brunt of the reaction. The further apart two metals are on a galvanic series, the greater the potential difference, and the faster the anode will corrode. All heating and air-conditioning piping systems handling water or steam should be protected from general corrosion by use of appropriate inhibitors. If dissimilar metals are in the system, an inhibitor system that protects both metals should be used. Proper engineering of these factors will prevent leaks from galvanic corrosion for many years. Lack of engineering and proper system maintenance will lead to disaster.

It all depends on the material being transported and environmental variables such as temperature. As mentioned before safety requires asking why you need dissimilar materials. Depending on the conditions of the system you might be ok with only the dielectric union.