As someone already pointed out. SL would erase the User folder if you use the Guest Account.
This isn't true, newer machines could not run an older OS. For example the Mac Classic could not run System 6.05, it required System 6.07 (or 6.08). Same with the PPCs, depending when they came out vs what the current version os was out, it would be able to run it. If it was close to the current OS it normally was like a .x release. The PPC could not run System 7.0 because, 1 there wasn't a installer for them in 7.0, and they had new ROMs. If I remember correctly the first PPC needed System 7.1 (Or was it System 7.5 I forget). (I've been using Macs since my SE/30 days.)
The point is, on the Mac newer machines have never been able to run an older OS on them. Back in the day as they say if you did manage to get an older OS installed. At boot up you would get 'This System Software Does not run on this machine.'.
Hugh