Ok, I have tried to install 10.5.8 to an existing 10.6.1 installation.
First of all you have to run disk-utility in 10.6.1
Here you can create a new partition, so the system has now 2 partitions.
The new paritition is named like OSXLEO
Ok, now you can install with an USB-Stick with a retail-copy of 10.5.6 to the second partition OSXLEO.
After the installation, you have to boot from the USB-Stick with 10.5.6 again. In the installation-menu you choose netbookInstaller and you have to run this application. Here it is important to install netbookinstaller (and chameleon) on the OSXLEO-partition (2nd partition).
Now, you can reboot your system.
If the blue-bar apears you have to press any key and here you can select the partition OSXLEO.
That's all :-)
Now you can install the 10.5.8 combo-update and after install you have to choose the OSXLEO-parition and you have also to type "-x".
OSX starts now in safety-mode, here you have to start netbookinstaller again.
Now I will see which version of OSX runs better (10.5.8 or 10.6.1)...
At the moment I'm not quite sure which version of the OSX runs better on the Mini10v.