Impossible to downgrade Mac Pro's operating system to original system
You do need Snow Leapord or Tiger and below to run PowerPC apps. That is how I also got into trouble, but in my case, my iMac is no longer usable.

I bought a spring 2008 iMAC that came with the Tiger OS which I upgraded to Snow Leapord. I also like Snow Leapord because it still runs all the old software based on technology no longer used.

After I upgraded to El Capitan, I too could not install an older release. Like you, not even with a new hard drive. 

I also found out, as you may have noticed, you can't install Sierra either. That is because of an upgrade to your current OS that tells it your hardware is obsolete.

I found this in the macrumors forum: "Apple Obsoletes Select Early 2008 to Late 2009 Macs"

Here is  "Apple's Official List of Vintage and Obsolete Products."

Both of our computers are on the list.

Since I tried to install Snow Leapord, my iMAC refuses to boot, even in safe mode.

However, I found this post on the Apple website. It is from 2013, and it may be able to help you. The article also provides the update link from Apple Support.

The post is "OS X 10.4/10.5 need to upgrade, 10.6.8 ok still" and it is by a level 7 user DS_Store.

I hope this helps, for all my attempts at dual boot disks, external boot disk, VM's and such may have doomed my iMac for good, which means 6-8 months before I can finish designing the apps I have been developing.