I was wondering if you can use the metonymy feather to refer to an angel(s)? Would feathers refer to several angels? Does this apply to poetry only or can I use it in other media?
Yes. But you could also use, respectfully meant, flying pigs. It all depends on what you want that name to convey, and what emotion you want to give the reader. The word nazi is a proud word to some, a hated name to others. Think about what you want the reader to feel and experience when reading that word. Then find one that best describes the emotion/feeling/message you want to convey.
Can you? Absolutely.
Will people immediately understand it? Probably not. This is not a common metonymy and feels a bit awkward to me, to be honest.
However, breaking the rules in poetry is commonly done. I would make sure to, in some way, explain what the metonymy means, somewhere early in the poem, and then call it good.
In other media, I would be cautious. This feels like it would start wandering into worldbuilding situations pretty quickly, which could be a good or a bad thing. I would be interested in a world where there's an explanation for why you can't call an angel an angel.
If Veronica Roth (of divergent fame) can work pompous, self-indulgent, SAT words like Amity, Dauntless, Erudite, Candor and Abnegation into her story, I am sure you can refer to angels as feathers.
"Who's the feather?"
"Dodos!!!!!!!" <--- for when angel cops showing up at a demon hideout... (if you want to stick with the bird theme)
Clearly... you would only use such words in a world where the actors are familiar with the appearance of the kid of angels you are speaking of.
Sorry.. missed the part about poetry.. I do not believe feather is commonly accepted reference to angels...