For a Mac Pro, probably the easiest option is to purchase an additional SATA hard drive and fit that to the Mac. You can then install Leopard from the DVD onto that new hard drive.
If you have MacBook Pro rather than a Mac Pro, then you can use Disk Utility to create an additional partition on the current drive. But you will need to create a backup of your current Lion partition first so that you can create two partitions on the existing hard drive, becasue doing so erases any existing partition. Personally I would purchase an external USB hard drive and with that formatted to Mac OS X Journaled, you could then install Leopard on that external drive.
If you already have a backup of your Lion partition then you can boot into the Recovery HD partition and open Disk Utility.
Select the hard drive with Lion installed on it and then select the Partition tab.
For the partition layout drop menu, select 2 Partitions
By default, the system will divide the hard drive into two even sizes. If you want to increase the size of the Lion partition, you can manually enter the size of drag the divider between the two partitions to the size you want. I would set 50Gig for the 10.5 partition and the remainder for Lion.
Click the Apply button and okay the selection
Once the partitions have been created, you can exit Disk Utility and select to Restore from Time Machine backup. This will let you browse to your Lion backup and select it.
Select the larger partition to install the backup to and then click OK to copy the data to this new partition. This will take several hours so let it run over night - as these things appear to take longer if you sit and watch them.
When Lion has finished being copied to the new partition, you can then insert the Leopard disk and reboot to the DVD and select to install Leopard on the smaller partition.
As for SANE, looking at the supported Canon scanners, your CS3200F is not listed so this software is not going to be a solution.