We're supposed to base it off of Ammianus Marcellinus and Tacitus, but does anyone know of any good secondary sources that would be worth a look? Additionally, should I stick to Tacitus' Annals, or are his other books worth a look?
Of course :) First of all, see if you can find a copy of Adrian Goldsworthy's The Complete Roman Army - it quite literally answers every part of your question. If you're looking for another primary source, check out Vegetius - his writings are incredibly descriptive.
I can suggest Hans Delbück's History of the Art of War series, most specifically Volume II: The Barbarian Invasions. The central theme seems right in your wheelhouse. That is, how the Roman military changed from the one that we know and love through the decline and fall of the Western Empire and the transition into Medieval Warfare.