Learn English – A collective noun when referring to a group of people with the same name

Naming has a large arbitrary and cultural component and so is subject to the same structures as fashion. It seems for many centuries the only choices for Western Europe were Christian saint's names and for men mostly just the apostles.

But fashions change. It seems like in 19th c. Americans, the fashionable source was Old Testament prophets with z's and k's (Ezekiel, Zebulon, Zachariah).

And lately (early 21st c) American girls are named after president's last names (the aforementioned Madison, plus Kennedy, Reagan, Taylor (this is the answer to one of the questions). This is simply a current fashion trend mostly likely to be replaced soon from some new set.

One subtle pattern in English speaking areas for at least the past two centuries is to name a boy using the maiden name of the mother, usually a second or third boy since the first got the full name of the father (with Jr. or III appended).

As trends go, people sometimes follow the external view of the phenomenon (last names for first), which accounts for Jefferson Davis or (much later) Harrison Ford, whose mother's maiden names did not supply their first names. That these first names are the last names of famous people (American presidents) probably helped in their choice.

I think you are correct, both semantically and in common usage. A useful retort to your wife's line of reasoning would be that no one would say "I am willing to exchange tickets for monies," even if the monetary exchange included different denominations, both bills and coins, or even several different types of currency.

A more academic argument would be that "metal" can already refer collectively to several types of metal.

OED b.1.b The constituent matter of a metal or of metals collectively; metallic substance.

This, combined with the previous example of how "money" is used collectively, I think more than justifies your usage.

On the other hand, referring to it as "metals" is not incorrect, and possesses as certain charm (as in how British English often sounds to Americans).