I am using an Apple Script to shuffle the photos in an album randomly, when I want to shuffle them for a random slideshow:
A quick and dirty way to see selected albums as a random slideshow is to collect the photos in an album and to use this album as the source of the screensaver. The Screensaver can shuffle the photos.
Only, in the most recent system versions the screensaver has a bug and can no longer show the edited versions. When you use a Photos album as the source, you will be seeing the unmodified originals.
It might be better to export the photos for your slideshow to a folder in the Finder and to use the folder as the source for the Screensaver. Then you can use third-party screensavers like the free ArtSaver to show your photos shuffled and with titles, dates, locations: http://www.cs.uni-bremen.de/~zach/software/ArtSaver/index.html