for my setting with 7″ touch and IQAudio Amp+, I used the image for the 7″ touch and installed the Plugin for my Soundcard. Thats works without any problems.
With the image for the IQAudio Soundcards, I had the problem, that the touch switched off after some time and it wasn´t possible to reactivate without external keyboard.
The HiFiBerry Digi+ Pro is already supported by our newest HiFiBerry Plugin, so there should be no problem. Have you already tried it the way orcas.cc has described it?