Downgrading firmware?

I have a program I'm hoping to run that is legacy and requires being on Leopard or other OSX versions around that era.

I bought a Mid-2011 iMac with hopes to just install Leopard - or Snow - or at least be able to run it from a bootable install on external. I have tons of old OSX installers on CD and also a few already installed bootable drives - many different versions. But I'm getting kernel panic crashes when trying. (See pic)

I'm 90% sure this is because the iMac had later versions of OSX installed - and new firmware updates along the way. Installing an old OSX might not be an issue but going back to the old firmware doesn't go along with it.

Any ideas on how to get a Mid-2011 completely back to the factory setup (including EFI)?