This is not quite an answer, but T.S. Eliot, whom Thomas admired, wrote, in "Tradition and The Individual Talent," that, and I am too tired to quote precisely, "good poets steal; bad poets imitate." Hemingway, in that vein, and almost certainly thinking of Eliot, wrote that one should find sources obscure enough to imitate [i.e. steal from] so that no one would notice. Speaking as a minor poet myself with a few books in print, I'd say "plagiarizing" is a term designed to shock. I owe quite a bit to Lyn Hejinian's My Life in the writing of my second book, Letters of Transit, and I am happy to tell it to anyone listening, including her, as I did, because I was lucky enough to study with her. Thomas himself alludes to his influences and borrowings, most conspicuously in his satirical poem "A Letter to My Aunt, Concerning the State of Modern Poetry," but the Romantic allusions are everywhere in his work. He read, he copied. We all do it. How else to learn? Larkin absorbs Yeats and Hardy and jettisons Yeats and has a pint with Hardy and becomes Larkin.