I think a brass faucet looks fine with a stainless sink. However if you're using brass, Ithink you have to pick it up elsewhere in the kitchen. The hardware is my preference, or in pendants or both.

GL, I love brass bridge faucets.

The sky's the limit with mixing metals these days. I think adding one more finish into the environment would actually be advisable as design choices (at least for accessories like that) seem more purposeful when done in odd numbers. What do they call it? The rule of three, I think? For jokes, for flower bushes, whatever- it always works.

Check out Lauren Liess's kitchen on her blog. Lots of metals and it's great.

I would check into having unlacquered finishes on marble. I recall one GW member, three apples I think, who had issues with the metal and stone together.

As for mixing finishes I think it will be beautiful. I have stainless/satin nickel for all my functional pieces and polished nickel for all my fancy bits like cup pulls and lights. I think it looks nice.