In order to get that electrolysis action, you need to complete the circuit.
Ground to Galvanized - ga;vanized to brass - brass to pex and you need an electrolyte between the brass and the galvanized. Water is kind of a poor conductor unless it is moist on the outside of the fitting.
So, if thereis water on the outside of the fitting, then a dialectric needs to be used.
If it's on a totally dry locaion, then doesn't likely matter.
The dialectric union is necessary, but further investigation, I believe is warranted in this case.
You need two disimilar metals, an electrolyte and a "circuit".
You have A. You may have B and you have a lousy C. Furthermore, the brass mipple will be insulated somewhat with the teflon tape.
For something that is going to be replaced, the extra expense might be avoided.