I really like both that Fleur de lis and Crown key, I think both are classic if you love them, and I don't even like Tiffany jewelry, but these don't scream Tiffany to me. They look more like vintage or Victorian jewelry. I totally got my key necklace out after seeing these and now I want to wear it tomorrow! I would only get one key, I don't really like the look of multiples together. I believe they are both an inch and a half long, so it comes down to which one you prefer and how you feel about the prices.
As for chain length, I saw that you can buy them w/o the chain which is what I would probably do and then just put it on a chain you already have or you could buy a few chains of different sizes elsewhere for the same price as that one chain from Tiffany's. Another option would be buying the shorter chain and then adding an extender to the back to make it different sizes. I do not think the key is too large for the 16'' chain, I measured my key for comparison and it is the same size as the ones you posted and it is on a 16'' chain with an up to 3.5'' extender. Mine looks great on the short and longer lengths depending on what I wear it with. Some necklines look better with a 16'' chain, some tops are not low cut enough to accomodate the 18'' and it competes. So I would figure out a way to have a little more versatility with chain length, either using chains you already own, buying multiple lengths, or using an extender.