Can we use an adjective made noun as singular? Incidentally, should it be 6 feet tall or 6 foot tall?

eg. He is a white and 6 feet tall.

I hear adjectives made noun are normally plural.
eg. The rich are helping the poor in their country.

Is it not possible to use it as singular like in the last sentence of the following?
eg. Once upon a time, there was a rich man and a poor man. The rich man was always helping the poor one. However, the poor was not appreciative of any help received from the rich.

I would very much appreciate your assistance here. Thanks.
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Comments  (Page 2) 
Or does 'awkwardness' mean my question could be uncomfortable and using the term 'white' could be offensive? I'm sorry if I used a bad example.
Yes, like it or not, the appropriateness of 'white' in your sentence colours (no pun intended) the 'naturalness' of the sentence, and helps confuse me, at least, as to whether such a word is 'right' or 'wrong'. The same confusion occurs (with me, at least) when I am asked to judge the 'correctness' of other students' sentences on topics in bodily functions, sexual procedures, and other personal areas. I cannot imagine their thought processes in choosing such.
I understand the point now. To me, honestly I was just interested in the correct usage of adjectival nouns, but I didn't know it was an awkward example. I just read the word in the newspaper and thought up of using it in this forum. I'd better be cautious next time and avoid using it.

Again, thank you for sharing your thoughts on this and your assistance.
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