7-8-9 speeds cassettes can all be put on the same hub. Since they have different
spacings between the individual cogs on the cassettes, the shifter will get out of
sync at some point depending on how you have it set up. Shifters are made to move
the RD enough to shift the chain precisely one cog spacing per notch, so a different
spacing will mean you are off a noticeable amount after 4-5 shifts and it will either
hunt between two cogs or be very noisy. If you set it up so the chain is centered
on the middle of the cassette, then the drift toward the in/outside will be less than if
you do the standard method of aligning with the smallest cog. Sheldon Brown discusses
a few other options on his website www.sheldonbrown.com.
8spd chains work ok on 7 and 9spd cogs, and frequently vice versa, if you are not
concerned about crispness (12-34 are never crisp) or a bit of noise.