1) Personally, I do not think all of the hardware has to match in finish. I have mixed chrome and brushed nickel and stainless in the same room. Personally, I prefer them to be in the same "family" (ie: silver colors), but that's just a personal preference. 2) I do not think that hardware finishes have a strong impact on whether a room looks dark or light. I believe that paint, floor color, countertop color, and vanity color have a much bigger impact on how dark a room feels because they fill up the space. 3) If you really like the contrast of dark wood (and/or dark granite or something) with the white, go for it. Consider using a light colored wall paint to keep the room from feeling too dark. I love the striking contrast of white and dark wood. 4) What other finishes would you like to mix in?

No the outside fixtures absolutely do Not have to match the door hardware.

I have a vessel sink bathroom with orb faucet, antique chrome tp holder, chrome trim on the toilet, and then an old toilet tank with verdigris and brass. So maybe I am not the person to ask! I think if you are going at all rustic, and the rest of the palette is muted, mixing is fine. You need to be more careful in a modern space or a very formal space or with "near misses", ie things that look like you tried to match but failed.

IMO it really depends upon individual situations. Proximity weighs in, too. Of course, all the same finishes is fine; but that does not mean mixing isn't to be done, just that it needs to be done thoughtfully. A mix can draw attention to what you want to be focal points and can make a room be more interesting visually. In our guest bath, all of the finishes are brushed nickel, except the TP holder. But the TP holder is attached to the side of a dark wood vanity, and I did not want to draw attention to it. The antique bronze blended in perfectly, yet does not look out of place because it is visually an extension of the vanity. Along those lines, shower curtain rods, shower door handles and toilet levers are often not a design feature you want to accent, so I would use what would make them visually less noticeable. Ceiling lighting and sconces generally are the same finish, but I believe you can successful pull off an outstanding antique bronze chandelier, for example, by having utilitarian sconces and faucet fixtures in a nickel finish. Items on the same impact level, such as faucets and towel bars, I would usually try to keep the same finish. If the faucets are the focal, they could be one finish and the balance of the other items in the room be something else. As for proximity, I believe all the finishes inside the shower can match, but be different than the finishes in other areas of the room, IF there is a reason/point of doing so. A shower head in one finish and the shower knobs in another would be trickier to pull off but may work if the finish of the knobs blended with the tile so the shower head becomes a focal point. Some finishes mix better than others. And the design style of the features in the mix matter. As does the backdrop, as mentioned above.