The indexing of the shifter is calibrated to the centre to centre sprocket spacing of the cassette. So long as the rear derailleur and the shifter are of the same manufacturer and are designed for each other, the rear derailleur doesn't know that it is operating an 8 speed or 9 speed cassette.
The individual sprocket width of 8 speed and 9 speed cassettes is virtually identical and virtually identical between SRAM and Shimano, so you can mix and match sprockets to make a custom cassette. You just need to use the correct spacers for the flat sprockets, however the smallest sprockets have a built in spacer, so you can't easily use those sprockets on an alternative cassette system.
The width of the rollers on 8 speed and 9 speed chains are identical, but 9 speed chains have thinner side plates to make the chain narrower in it's overall width. For this reason you can use 9 speed chains on an 8 speed sprocket spacing but you can't use 8 speed chain on a 9 speed sprocket system.
Single speed chain uses a roller width of
3.18mm or 1/8
6 and 7 speed chain uses a roller width of
2.3mm or 3/32
8 and 9 speed chain uses a roller width of
2.2mm or 11/128
10 speed chain uses a roller width of
2.2mm or 11/128
Single speed chain has an overall width of
8.8mm (determined by the pin length)
6 and 7 speed chain has an overall width of
7.4mm (determined by the pin length)
8 speed chain has an overall width of
7.2mm (determined by the pin length)
9 speed chain has an overall width of
6.6mm (determined by the pin length)
10 speed chain has an overall width of
5.9mm (determined by the pin length)
Shimano 8-speed Centre to centre spacing =
4.8 mm
................................Sprocket thickness =
1.8 mm
...................................Spacer thickness =
3.0 mm
SRAM 8-speed.....Centre to centre spacing =
4.8 mm
................................Sprocket thickness =
1.8 mm
...................................Spacer thickness =
3.0 mm
Shimano 9-speed Centre to centre spacing =
4.34 mm
...............................Sprocket thickness =
1.78 mm
..................................Spacer thickness =
2.56 mm
SRAM 9-speed....Centre to centre spacing =
4.34 mm
...............................Sprocket thickness =
1.80 mm
..................................Spacer thickness =
2.54 mm
It starts to get a bit messy with 11 speed chains.
These spacers give you options.
http://problemsolversbike.com/products/cassette_spacers
