Are nominal adjectives and fused-head NP just two different explanations for the same thing?

Are nominal adjectives and fused-head NP (e.g. "the poor") just two different ways of describing the same thing, or is one considered a subset of the other, or are they different types of classification? Aren’t both ways of describing nominalization occurrences?

Or does nominal adjective just refer to the adjectival portion and a fused-head construction a more general classification that can include accompanying determiners?

Further, and related, is "beginning" (as in, "it was just the beginning") an example of using a present participle as an adjective and then as a nominal adjective/fused-head NP.