You are limited to the 12 in the list, but there's no reason why you can't
use "Customize Current Pen" to change the properties of the pens such that
the breakdown between "pens" and "highlighters" is something other than 8&
4.
I only need 1 highlighter, so I have customized the other 3 to act as pens.
I had a problem when I tried converting a pen earlier that I could not read
through the ink.
I believe however that you are speaking changing the pen/highlighter divider
location. I will see what I can do there. That may fix the opacity issue.
Thanks!