Maybe he met her in passing.
OK, apologies in advance, but my innate laziness precludes doing a bunch of quotes, so . . .
Elthir, it sounds like whatever source you read was talking about
cuir bouilli -- which wasn't really "boiled", but was a hardened leather, and, though it was used as a kind of cheap and light
armor from ancient times into the Medieval (and into the 20th century, in some cases, the Germans not trading in their pickelhaubes for stahlhelms until 1916 -- leather not proving very good against bullets and shrapnel), I'm not aware of any specific references in
Tolkien, though I could well be wrong there.
Tar Elenion, thanks for the citations. I never considered the iron collar to be "plate", but I suppose it's a matter of opinion.
For scales, besides the Southron, Tolkien talks
somewhere (laziness again) about the
Isengard orcs having only the "heavy and clumsy armor they made for themselves"; there may have been a "crude" thrown in there too.* I always took this to indicate scale, as this is much easier to make than mail, and likely of iron, rather than steel. And in fact, there's a passage in the drafts, where
Frodo and
Sam are escaping from what would eventually become
Minas Morgul, when they encounter an
orc in scale armor; IIRC Frodo stabs him up under the the plates.
I believe the
Ulmo references are to mail, not scale, as Tolkien, who would certainly know the difference, calls it mail in one of your citations. I think it's used to give a more piscine feeling.
The fish-scale simile also goes for the other quotations, I believe; I think they're meant to emphasize the quality of the workmanship -- it's suppleness and intricacy. Mail is made in various ways, some fairly simple, some highly complex, aa can be seen here:
Mail made with smaller rings and more connections obviously takes more time and skill, and could well be seen as like "fish scales".
As a somewhat irrelevant end note, I'll mention that a Sumerian myth, that passed on through Akkad to Babylon, talks about wise "fish-men", or "fish-gods" who came out of the sea to grant knowledge and protection to the first people.
I could tell a story connected with that, but it would be even more off topic.
So I won't.
* Late edit: Found it while looking for something else:
In Isengard as yet only the heavy and clumsy mail of the Orcs was made, by them for their own uses.
Unfinished
Tales, p. 366
So maybe it wasn't scale, after all. Or maybe he was using the term on a more "generic" sense.