No, this will not work. You can have multiple versions of OS X on one machine, but you can only install newer versions than the one that came pre-installed. For example, if your machine came with 10.6.4, you can't run 10.6.3 or older.
When Apple releases new hardware, the drivers for that hardware get rolled into a special build of OS X that comes only on that model. They don't get added to OS X in general until the next point release. (That means that drivers for the hypothetical machine I'm talking about wouldn't be added until 10.6.5.)
Apple doesn't release drivers as standalone software, which effectively makes it impossible to install and older version of OS X than what your Mac shipped with. You'll find that if you put the install disc for an older version in the drive, it either won't boot or it will refuse to install.
However, you can go the other direction. If you have an old Mac running 10.5, you can use Disk Utility to make another partition (HFS+ Journaled), then boot the install DVD for 10.6 and install it to that partition. To switch between the two after installing, hold alt when you boot up. (You can also select the default boot partition through the Startup Drive pane in System Preferences within either OS X install.) Boot camp is not required for this. Boot camp is only required for installing non-Apple operating systems like Windows and Linux.
The above [now below as this is the 'accepted' answer] instructions will install Snow Leopard - but not technically a clean install.
To do so follow the above steps but enter Disk Utility after choosing the language in the installer. You can wipe it from there.
Stolen from Macs.com
the Snow Leopard installer will ask which language you want to use as the main language. Make your selection and click the right arrow key.
The Install Mac OS X screen will display. Click the ‘Utilities’ button.
In the Apple menu bar, select ‘Disk Utilities’ from the Utilities menu.
Disk Utilities will launch. Select one of the following instructions, depending on what you wish to do.
Erase a Volume. Use these instructions to erase an entire volume. Be sure to back up all of your data first.
Format a Hard Drive. Use these instructions if you wish to erase an entire hard drive, including any volumes/partitions it may contain, and not create any new volumes/partitions. Be sure to back up all of your data first.
Partition a Hard Drive. Use these instructions if you wish to erase and partition a hard drive. Be sure to back up all of your data first.
When you have finished using Disk Utility, select ‘Quit’ from the Disk Utility menu.
You will be returned to the Snow Leopard Installer to continue the installation.
Best Answer
Snow Leopard requires about 5 GB of space to install itself and will run with not much more space than that total in most cases.
Installing on a low capacity flash drive (USB storage) is possible, but very slow since the chips there are optimized for low power and react far, far slower than even a basic, bargain basement spindle hard drive.