The "rule" is that it's a matter of count vs. mass.
The question is whether it's a really a rule at all. It seems to have started with Robert Baker in 1770, who expressed it clearly as a preference rather than a rule. It didn't become a rule until some time after.
On the one hand, this is a good argument to just use either, as you see fit.
On the other, there are cases where less is ambiguous and fewer is not. "Less people" isn't ambiguous, but "less excellent people" is, if you mean that the number of people was less, but their excellence just as high.
There being a "rule" that many don't follow, is enough in itself to make fewer more formal and less less so, in those circumstances where you could use either.
It's hence probably advisable to favour fewer in formal writing. Whether you always do so or use whichever sounds better to you in other cases, will be a matter of opinion, without hope for consensus.
"Fewer than one person" and "less than one person" both sound strange though, when nobody is probably going to serve better than either.