Can iMac 5 (Retina 5k, 27-inch, Late 2014) be downgraded if it comes factory-installed with Yosemite?
It is impossible to downgrade this Mac to anything less than Yosemite - the hardware forbids it. You have proved this in practice by getting the No Boot sign.

As for Pro Tools: from the Avid site: Note: Pro Tools 10.x and earlier (including Pro Tools Express) will not be tested/qualified with Yosemite (or Mavericks).

You will have to upgrade your Pro Tools to version 11 if you want to use Yosemite.