Yes, it was 9 months ago. Let me think. I don't have the settings anymore because I had to replace my harddrive :(
I actually set up Automator to do two different slideshows.
One in iPhoto and one in aperture.
Since iPhoto won't load new icloud photos during a slideshow I made it to where iPhoto would run and then when it was done aperture would load another slideshow and iPhoto would update in the background. Then the cycle would restart.
I had it to where automated would trigger when the icloud folder had new content loaded to it, iPhoto's slideshow would start. Then an automator timer would countdown, then trigger finder to copy the iPhoto icloud folder contents to the aperture folder during the iPhoto slideshow. When it was done copying (or timer based, can't remember) Automator stopped the iPhoto slideshow, and started the aperture slideshow, allowing time for iPhoto to download more new pictures. I had Automator set to five minutes and it would turn the aperture slideshow off and then start the iPhoto slideshow again and it would start over.
You'll need to put everyone on to an icloud account you set up (and make sure that you take them off or they do, I was receiving some...interesting photos unknowingly for months and then found them later...)
There may be much easier or better ways to do this, or programs that do it now. I had 4 days to figure it out and so I didn't have a lot of research time.