Struggling to downgrade from Mountain Lion to Snow Leopard

Hello,

I'm attempting to downgrade a Mac Pro from Mountain Lion to Snow Leopard as Mountain isn't compatible with some of the software on the machine. The machine was sent in for repair as it kept crashing when trying to render in After Effects. The machine came back after being completely wiped with Mountain Lion (an upgrade from the original Snow Leopard) and with a new internal hard drive that seems to have created two partitions.

My colleague has since wiped the computer again to try and reinstall Snow Leopard and apart from being unable to boot from the Snow Leopard installation disk there seems to be no other problems or error messages. I have tried holding Alt when starting up and it doesn't show anything other than a recovery drive. I've tried holding C and the computer freezes on the starting Apple icon so I'm unable to boot straight from the disk.

I've also tried to copy the installer onto an external hard drive but again, it isn't recognised when hitting any relevant keys during start up. However, both the disk and drive are recognised once in Utilities. I can't go back to Mountain Lion as a temporary fix as I receive an error message saying that it's unavailable. The computer is stuck in limbo and I'm desperately unsure of what to do next. Any assistance would be very appreciated.

My rough estimate of the specs is below (they're aren't exact I'm afraid).

Intro Date: July 27, 2010* Disc Date: June 11, 2012 Order No: BTO/CTO Model No: A1289 (EMC 2314-2) Subfamily: Mid-2010 Model ID: MacPro5,1 Std RAM: 6 GB Std VRAM: 1 GB Std Storage: 1 TB (7200 RPM) Std Optical: 18X DL "SuperDrive"

Many thanks.