If iron couldn't be stacked ... I hate to think of it! I don't know how I would store it, I would have no room to work, or the walls would be covered with iron.
Oh, that's right, the walls are getting covered with iron ... no more room to stack them. LOL!
Put a 'buffer' between the pans? Tried it once, just a waste of paper. If you only wipe the pan before putting it back in the stack your buffer material will get oily, and remove the oil, even if the buffer is non-absorbent. If you wash your pans (weak dish soap solution) there is nothing (sort of) to get on the bottom of the pan stacked in the pan.
Scratching the pans ... I'm sorry, I have to chuckle ... I don't even worry about seasoning iron. I'll burn out a pan, let it cool enough so it doesn't burst into flames when I throw some "oil" or grease in it to keep it from rusting, then put it in the stack. If it seasons it seasons, if it doesn't season it goes in the Garage Stack. The thought of scratching Iron by stacking it never crossed my mind. But then I'm a tactile person, It's in my genes or teachings that I don't bang things around.
Put something under the stack to protect the shelf? Most cretinly, use something cheap and disposable or easy to wash.
NOW, if it's enameled iron, that's a different creature .... I still stack it, but it looks better on the wall. L