Yes, just like with many distros you can add external repositories. Nonguix is a repository with non-free software like the linux kernel and binary firmware
Ignore the weird wording about not promoting it on the page, it's just a warning to keep people from complaining about the nonguix packages to the guix devs, but there's lots of crossover
Unless I'm missing something, Plasma is the KDE desktop, there isn't a separate desktop environment just called KDE... In fact, it's full name is KDE Plasma, it's just frequently shortened to either KDE or Plasma
KDE separated out into Plasma and the utilities and apps separately at KDE 5. Previously it was all one package, so there is a bit of a confusing distinction.