How will icloud photos deal with devices/pcs with low storage space?

[iPhone7ate9]

My laptop has 1gb of room, and my ipad only has 1gb of room. Say I take 20gb worth of pictures with my iphone 6, what happens when it tries to push all the pics, even smaller versions, to my other devices? And then if I delete them on those devices they get removed from all devices right?

 

[BrettDS]

My wife has about 25G of photos in her icloud photo library and photos takes a bit under 2G on her ipad where she has it configured to optimize device space. So you'll probably need to free up some space on your devices to get the smaller versions to load there, but you won't have to free up too much.