Q: mac pro booting from older OS

No. The latest Mac Pro cannot run any OS X older than Mavericks

You might be able to run it if you run it as a virtual machine with VM software like Parallels or VirualBox. You need a clean install of 10.8

Actually I just bought a 2013 mac pro and the earliest OS X it boots is yosemite.

Smokerz-

The Mac Pro late 2013 hardware is unchanged since its original release, so if you wanted to bad enough, you should be able to run the second minor release of Mavericks, 10.9.1, or later 10.9. Whether you can actually obtain that software is a different issue.

This says iWeb 3.0.4 works on Yosemite

http://jeffnitschke.com/wordpress/2014/10/iweb-still-working-yosemite/#.VJWcBl4A AA

Unfortunately he cannot run 10.8 in a virtual machine with a newer OS X version as the host, because that is a violation of the 10.8 software license agreement. If it's only iWeb he needs, no matter; it runs on 10.10.

Grant,

I just bought a 2013 Mac Pro with Boot Rom ending in 07. It will not boot a USB3 or Thunderbolt connected external with Mavericks as late as 10.9.5 It boots straight into Yosemite every time. That's the internal drive. It will not take a Mavericks install of 10.9.5 either. 10.9.5 is much later than 10.9.1 special build and should work.

I also have a 2013 Mac Pro I bought in early 2014 with Bot rom ending in 04, this machine does boot and take a Mavericks install.

If you still disagree maybe I should return the latest 2013 Mac Pro.

Apple documentation does not yet reflect that version. Let's hope it contains some [working] special sauce that will fix lots of subtle problems, and will soon be propagated to the existing Mac Pros.

This is the "latest" versions reflected in documentation:

Mac Pro
Mac Pro (Late 2013)	MacPro6,1	MP61.0116.B04 (EFI 2.0)	2.20f18 (SMC 2.0)
Mac Pro (Mid 2010)	MacPro5,1	MP51.007F.B03 (EFI 1.5)
Mac Pro (Early 2009)	MacPro4,1	MP41.0081.B07 (EFI 1.4)
Mac Pro (Early 2008)	MacPro3,1	MP31.006C.B05 (EFI 1.3)
Mac Pro (8-core)	MacPro2,1	MP21.007F.B06 (EFI 1.2)	1.15f3 (SMC 1.1)
Mac Pro (Original)	MacPro1,1	MP11.005C.B08 (EFI 1.2)	1.7f10 (SMC 1.1)

from:

EFI and SMC firmware updates for Intel-based Macs - Apple Support

Yesterday I went to Apple Store and Microcenter. Any 2013 Mac Pro with EFI 04 booted Mavericks from my external drive. Same at Microcenter. Seems the Quads are 04 and sixes are 07. I know there is no current documentation at Apple support that I could find about 07, other than hands on real life experience.