If the galvy bolts are scratched they may indeed start to rust. That can be dealt with by painting. Steel
are about as far appart as zinc (galv.) and aluminum
. Nowhere near as far appart as stainless and aluminum. The aluminum won't suffer much with steel
but with stainless it will. I just purchased an aluminum boat
which has been modified over the years. Everwhere there is a stainless bolt or other fitting there is the cruty, bubbly aluminum crud. The stainless will be the first to go! Granted, the stainless nuts and bolts have little problem being undone but the aluminum is not fairing well where stainless is used. The steel bolts and fittings are harder to undo but the aluminum is like new around them.
With a windlass
mounting plate you probably have plenty of sacraficial thickness and the thing is readily replaceble so do what you like. But if you look at the galvanic scale you will see that stainless is at the opposite end, closer to copper, and the aluminum will suffer. Whatever you use, protect the aluminum where it might hold moisture and be starved of oxygen, like the bolt holes and underside, by painting imediately after removing the aluminum oxide (ussually by sanding) to try and prevent crevas corrossion. (Stainless also corrodes where oxygen isn't pressent).