The preposition 'about' before numbers

It is odd to say "About one worker". It would be normal to say "About 100 workers", to mean between 90 and 110 (or more, or less, in context)

If you mean a small but non-specific number use "a few":

The company employs a few contract workers.

It would be fine to say "about 1 metre" which I would understand to be in the range 0.9—1.1 m (or perhaps a smaller or larger range, in context). But I would not call 2m to be "about 1m". Similarly, I would not call "2 workers" to be "about 1 worker". This makes "about 1 worker" a rather odd expression.

Similarly "about fifty to a hundred workers" is fine, but "about one or two" is odd. Just use "a few" in this situation.

Can the preposition 'about' be used before numbers like one or two? Example:

It employs about one contractual workers in the business.

Yes. Although this is a bit casual for such a business discussion. Colloquially, very common.

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Can the preposition 'about' be used before a range of numbers like one to two? Example:

It employs about one to two contractual workers in the business.

I would say avoid about here and use around, or the like (between-and, from-to). However, in very colloquial informal speech, you could also use about.