I looked at your art, and I think that that conclusion to draw is as above: they are a third 'grade'. As you said, their legs are digitigrade form, but their feet are plantigrade. I'll look into it.
EDIT: I looked it up, and there are only three grades, none of which are similar to Protoss. (Digitigrade, Unguligrade, and plantigrade.) Note, however, that in my original post, I specified Protoss type legs, which is really the part we agree to be digitigrade.