The default install, not checking any additional packages, is the minimal install and should be around 100 MB as covered here. I found this cygwin-lite which looks interesting.
pdfmom is not a standard utility in Cygwin. The groff source package includes pdfmom.pl, but the binary package doesn't include it.
You may have luck bringing this up on the Cygwin mailing list and requesting the groff package maintainer to add pdfmom.
First off, I hope you've been looking at the official cygwin docs because they really are excellent imo. Very in depth, if a bit dense. This page in particular is helpful.
However, I'm hoping this will solve your problem:
>18.4 CygWin >The CygWin interface is similar to the Linux interface (18.3), i.e. console input and X11 graphics output. To avoid the warning of a missing graphical user interface, you have to start the X11 window manager by issuing the commands >$ export DISPLAY=:0.0 >$ xwin -multiwindow -clipboard & inside of the CygWin window before starting ngspice.
From here. Btw that page has a manual table of contents that you need to search with your browser; I'm sure you could figure it out but I found it neat.
If that doesn't work to get the ngspice X window, hopefully somewhere in that section of the ones around it (where the compilation info is) will help you compile it again linking to the X libs. Reply back if you have issues.