Originally Posted by
IthaDan
Right, 7 and 8 speed have the same spacing BETWEEN the cogs, but 8, 9, and 10 use a wider freehub body and have unique spacings between the cogs (and all have unique chain widths). if you have the right shifters and chain for the cog spacing, and don't want to swap an 8 speed+ width freehub body onto old wheels, you can go one speed down on a 7 speed freehub width. That is make an 8 speed out of a 9 speed cassette minus a cog and use a 9 speed shifter and chain.
EDIT: for the record the cogs ARE thinner, but the pin to pin distance on the chain is the same, so a wider chain will work if you don't have to shift it, or it's in the front where the size difference is so dramatic.
I'm not sure if I'm misunderstanding you but I think you have a slight error.
7S and 8S spacing is just a tiny bit different. 7S Shimano is 5mm and 8S Shimano is 4.8mm. So you can sometimes go back and forth with the 7/8 shifters/cassettes, but it does not always play well. It can be tricky to get it setup just right if you mismatch.