Is galvanized steel totally incompatible with cedar, even when only touching on the surface?
Yes, to both. The cedar will last for decades and your hardware should too. Simply isolate the galvanized from the stainless and cedar, but definitely don't trade the cedar for some much lesser grade wood. Don't get talked into hot-dipped fasteners, stainless is the only way to go.

Buy thick fiber or plastic washers or even use cut-up pieces of tar-paper, rubber, roofing shingles, milk jugs or soda bottles. Of course, the more UV resistant the better, the exposed cheap stuff can be trimmed or spot painted after installation for UV protection.