What is the smallest size font we can print?

Font size is definitely not the issue for small - but your goal of .018 wide is unreadable to the human eye. The photo above was taken through a microscope.

At a typical kerf (the width of the laser beam) you’re going to get fewer than 6 strokes total. If that size is your requirement you might as well just score a black box.

A score is literally a cut that doesn’t make it all the way through - so it will definitely be darker than an engrave which is a discolouration of the surface material (unless you have it on vary power in which case it’s cutting it away) that covers a wider surface area behind.