About contact corrosion between the galvanised bolts-nuts-washers and the stainless steel flanges. Is it not neccesary to worry about this, or are measurements needed (for instance: stainless steel stud-bolts & nuts, plastic washers, protection against electrolite (rainwater)).
Apart from the galvanic corrosions issues, that others would be better to answer than myself, note that the worldwide price of galvanized bolts, type 'hot dipped galvanized' has increased considerably the last years, and there should be no obvious reason not to specify nuts and bolts in the same material as the flange material if flanges are of ss quality.
The importance and also quality of protection of galvanic corrosion will depend on cost consequences if failure: it is more important to protect a deep-see oil/gas pipeline or construction against possible galvanic corrosion (all measures taken, selecting comparable materials) compared to for instance an above-ground less important/uncritical water pipeline.
Anyway reasonable precautions on land usually do not need to cost much, but should be taken, as you point out.
Also note that in water treatment plants and waste water treatment the use of SS pipelines are common (At least in Europe), while valves often are nodular cast iron or cast iron epoxy coated, and depending on  check valve construction, conection bolts are fairly often stud-bolt types.
You have then the opposite problem: SS studbolts in cast iron houses.

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