"The composer is a tall English man."
Translation:A zeneszerző egy magas angol férfi.
Is the indefinite article "egy" required here? (It certainly clarifies in writing.)
Without egy it doesn't feel incorrect to me, but a bit less natural. I think it's better if it's there.
In a word, yes. It can be omitted directly before a noun, especially an occupation, but not if the noun is qualified by an adjective.
Is "férfi" strictly needed here? I believe in the past adjectives like "fiatal" and "piros" have been used as nouns; does that work with "angol" too?
Yes, angol could be used as a noun here. (But of course the English sentence would be "The composer is a tall Englishman" in that case.)
You are right, as angol doesn't indicate the gender. I wanted to emphasise that in the alternative sentence it's a noun and not an adjective, but on second thought, it's not necessary for the translation to be equivalent in that sense.