Your most common alloys of gold are with copper (rose gold) and silver (electrum, but this is actually another way to get "white gold" (and white gold is another name for electrum), electrum is important only in that it is actually a naturally-occuring alloy, so the notion of purposefully alloying electrum is a little odd).
"White gold" usually refers to other gold alloys, such as those of palladium (commonly used these days) or nickel (less used now due to reasons including allergic reactions in some people). You mentioned nickel, but yeah, other things are used these days, not that that matters for DF.
Most gold we use in the real world is some kind of alloy of gold, silver, and copper all at once. A good measure would be 2 gold, 1 silver, 1 copper, which would make 12 carat gold.
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Wait, silver too? Source?
Since mixing in copper makes gold more reddish (hence, rose gold), you can actually mix copper in with white metals like zinc or nickel, and produce a somewhat more yellowish gold than the white gold formula above. Something like 3 gold, 1 nickel, 1 copper.
For what it's worth, zinc wasn't isolated until relatively recently; its existence as an elemental metal in DF is highly anachronistic.
Oh, hey, and Black Bronze, according to Wikipedia is 96% copper and 4% gold, but in DF is 2 copper to 1 gold to 1 silver (no idea how silver got in there)
You mentioned Toady's notes about Corinthian bronze; that's where the silver comes from.
Regarding gold furniture, another problem is:
That's a lot of fucking gold. If you have that much gold lying around that you're making furniture out of it, you're doing pretty well for yourself. I could see some kind of ridiculous gold throne being made for a king, but you'd have to be pretty careful with it. I don't know that strict restrictions should be in place except for maybe objects that you know will see heavy use.