I still don't see how I'll avoid cellular data charges through the WiFi hotspot.
I need to keep the hotspot on in my pocket to get emails and other low bandwidth traffic (like occasional use of Safari, travel apps) to my iPhone. So, as soon as I take some pictures, all that MB's of data will get transferred to the iCloud photo library through the cellular hotspot.
It appears that there is no way to suspend iCloud Photo Library uploads (without cancelling the feature and losing all my pictures on my iPhone) while connected to WiFi
