This is interesting. I can only respond with my own related idea, which is that the mind only thinks in terms of unities -- a priori. We can see a book as one book or as many chapters. We can see a chapter as one chapter or as many sentences. And so on. It's something like a zoom lens.

So everything thing that is complex, having several properties, would be a unified compound. But then one could project/consider the unity of the components involved, and one can then look at the components as compounds.

Perhaps the minimal unity is the concept of nothingness, which is a unified absence of properties, or the negation of all properties exception the un-negatable a priori intuition/concept of unity.