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davidcyclebackA potential problem with stacking photos on top of each other is that the image surfaces can stick. This is more of a problem with modern photos, especially color photos, which still have moisture. Most 1800s photos are pretty much dry as a bone and it's not a big issue. Another possible problem with stacked photos is scratches to the images-- though if the photos are in individual holders, even soft sleaves, it's not much of a worry.
I've had lots of photos, old to new. I've had only a few instances of photos sticking to each other. These were modern photos (1990s- ), usually color, and the photos were wrongly stacked image to image. Make sure, photos are stacked back to front if they aren't in a holder.
I have a lot of large modern paper photos (no cardboard backing). My biggest problem is dinged and creases to corners and scratches to the mages. I find that the best thing is to put the photo is a penny sleave-style holder and stack them carefully and out of the way, so I don't step on them.