Substantial Unities

If a Dominican defends transubstantiation to the point of re-affirming that bread is one substance (even though it isn't), is he being a good follower of Thomas, a good Tridentine Latin, or both?

Rhetorical question.

Mereology

How underdeveloped is mereology (the study of parts) by neo-Thomists and neo-Aristotelians in their physics and metaphysics and their understanding of substance?