I suggest you get your photos on your iDevices copied over to a PC, just in case iCloud photo does something weird. I turned on iCloud photo library on my iPad mini and lost a whole bunch of photos which I could not see on iCloud.com. It only uploaded the 4 pictures I had in camera roll. Fortunately, I have a backup of all those pictures that were on the iPad, so I'm not completely mad 
Also, I found that when I connected my iPad to my PC to copy off the photos before trying this mess, I could not see all the photos. I used iExplorer to get all the photos off. It recreates all your folders from your iDevice on your PC. There may be other free programs, but this one was $35 and I feel it was worth it.
I just want to reiterate for you to copy off your photos before playing with iCloud photo library. The idea is good, but there are bugs and it will take a while to get them ironed out. Don't lose your photos over this.
Also, I found that when I connected my iPad to my PC to copy off the photos before trying this mess, I could not see all the photos. I used iExplorer to get all the photos off. It recreates all your folders from your iDevice on your PC. There may be other free programs, but this one was $35 and I feel it was worth it.
I just want to reiterate for you to copy off your photos before playing with iCloud photo library. The idea is good, but there are bugs and it will take a while to get them ironed out. Don't lose your photos over this.