I was curious as to whether or not St. Thomas Aquinas every formally laid out any principles to be adhered to when writing/studying/interpreting history or ever elucidated a theory of the "nature" of history itself.
About historiography as a science, I am not aware of any statements from Thomas Aquinas. In the absence of others finding a better treatment, here is the most direct mention I could find.
In ST I, Q. 1, a. 10, "Whether in Holy Scripture a word may have several senses?", he includes a brief account of the literal sense of scripture, under which he includes a brief definition of its three components: history, etiology, and analogy.
The first two certainly belong to the idea of historiography:
illa tria, historia, aetiologia, analogia, ad unum litteralem sensum pertinent. Nam historia est, ut ipse Augustinus exponit, cum simpliciter aliquid proponitur, aetiologia vero, cum causa dicti assignatur, sicut cum dominus assignavit causam quare Moyses permisit licentiam repudiandi uxores, scilicet propter duritiam cordis ipsorum, Matt. XIX, analogia vero est, cum veritas unius Scripturae ostenditur veritati alterius non repugnare.
These three, history, etiology, and analogy, pertain to the one literal sense. For history, as Augustine expounds, occurs when something is presented on its own, while etiology is when the cause of the event is assigned, as when the Lord assigned a cause for why Moses permitted a bill of divorce, namely, because of the hardness of their hearts (Matt 19). It is analogy, on the other hand, when the truth of one scripture passage is shown not to be in conflict with the truth of another.
It is also worth noting that he cites Eusebius's Ecclesiastical History in a few places, so he certainly judged it worth reading.