Finest ballpoint/rollerball pen?
Ball tips

I was familiar with the Uni-ball Signo 0.28mm tips (virtually the same as a 0.3mm tip, which is commonly available), but the 0.18 was new to me, and that's apparently been discontinued.  I suspect that's pushing the physical limit of what you can do with a ball tip.  

A number of manufacturers have 0.25mm rollerballs.  If you've found one with a 0.2mm line, you will have a hard time doing better than that.  Note, though, that even the .25mm ball pens have a reputation for clogging, and ball tips that small can be scratchy to write with.

Ball diameter vs. line width

To answer your question about ball diameter vs. line width, the ball needs to be captured at the end of a metal tube, so just a small portion of the ball sticks out.  That portion will be smaller than the diameter of the ball, and that's the part that defines the line width.  

That line width spec isn't always a reliable indicator of line width.  If the tip dispenses a lot of ink, the line can spread and end up wider, depending on the media you write on.  Also, the softer the media and the harder you press, the more of the exposed portion of the ball will contact the paper.  So different pens that all claim to write a specific line width will have very different lines.  

Brand

Brand does matter.  Once you get beyond manufacturer's that exaggerate, and are dealing with products accurately described, there are big differences in manufacturing quality and in how long the pens will last.  There are also differences in writing feel, comfort, and line quality.  Different people swear by different brands.  Some of that is due to the type of writing and media, and some is personal preference.  You can check reviews and opinions to narrow the selection, then try some different ones and see what you like.

Super fine lines

The only kind of pen I've seen that claims to get down to 0.05mm or less is a plastic or felt tip pen, and you're already familiar with those.

If you want to go smaller than you can with a ball tip, technical pens fill the gap in that size range.  They're called technical pens because the tip is contained in a thin metal tube that allows it to be used accurately with a straightedge or template.  There are several types of tips used in these.  One type is similar to your Fineliner; the writing tip is plastic or felt.   

The other is a metal tip; it's a tiny metal capillary tube that can get down to very fine line widths.  A Rapidograph 6x0 tip has a 0.13mm line.  A Staedtler MarsMatic 5x0 tip is 0.13mm.  Several manufacturers make a 0.1mm tip.  The pens and tips are available from art and office supply stores, and online.  

Even with technical pens, the claims of lines finer than 0.1mm (0.05mm and even 0.03mm), are for a plastic or felt tip.  Those claims can't actually be true.  For perspective, A single dot of inkjet printer ink is in the ballpark of 50 microns.  A single dot of toner from a laser printer is in the ballpark of 80 microns.  Dots this small are barely visible up close to the naked eye if they are arranged in a regular pattern.  To create a line that narrow with a pen, you would need to write with a single human hair, feeding ink through its hollowed out center.  So claims of writing a 50 or 30 micron line with a plastic or felt tip just aren't realistic.

I've used a 0.13mm (Staedtler MarsMatic) tip.  It's easy to use on a surface like velum or tracing paper, but it's like trying to write with a needle on regular paper.  At the small end of this style, you need to develop some skill with it to use it on regular paper (which those sizes aren't designed for).  Sizes approaching the smallest ball-type pens aren't a problem writing on paper.  As a practical matter, if you want to make lines substantially finer than what a ball pen can do, and want to do that on regular paper, a plastic or felt tip will probably be the best solution.