JTam... The Roman history offers clearly a lot of possibilities.
Advise on making your choice ; start with browsing "Timeline of Roman history" on Wikipedia. You will spot the wars and peoples concerned with a quick eye, century by century. If you see an era that particularly speaks to you, then you will be able to go further...
If you decide, for example, to work on the 3rd/2nd century BCE, you'll chose a core of Victrix Republican Romans...
For the 1st century BCE, Warlord Romans will be the good basic line for you (mixed with Victrix Rep. Romans wearing chainmails, possible conversions)...
And for the 1st century AD, Victrix Imperial legionaries and auxiliaries will be OK.
Rome's allies and enemies now : Gauls/Celts are usable mainly for the 1st and 2nd period, and as Britons for the 3rd ; Hellenistic warriors are usable for the 1st and the 2nd period (Hellenistic kingdoms, Pontus, Palestina) ; Germanic tribes are usable for the 2nd and the 3rd period ; Iberians for the 1st and the 2nd ; Dacians for the 2nd (as Illyrians) and the 3rd, and so on...
Mix are sometimes possible : Cimbrians and Teutons were a mix of Germanic and Celtic peoples together, Belgae idem ; Celtiberians were a mix between Celts and Iberians... A Jewish army (4 interesting periods are possible : c. -150 (revolt of the Maccabees), c. -63 (Roman 1st siege of Jerusalem), 60-70 AD (Zealots, Masada), and Bar Kokhba revolt (c. 135, destruction of Jerusalem) can be a mix with Hellenistic, Oriental and Roman elements of the time, plus civilian ones...
You can work only with Romans (2 armies as a core), adding ethnic auxiliary units to evoque the "clientèle" (the area of influence) of the Imperator. For example : the Caesar's army of the civil war was constituted with a core of Roman citizens (1/2), plus Gaulish and Germanic warriors (1/2) ; the Pompeius Magnus' army was recruited among Roman citizens (1/2), plus Hellenistic and Spanish warriors (1/2).
The possibilities are wide. Osprey covers the entire period (Warrior, Elite, Combat or Men-at-Arms) : excellent basic source, as those who write for Osprey are often recognized specialists, historians and archaeologists. When you will make your first choice, read "Roman Legionary" (dates, according to the period you're in) and "Rome's enemies" (according to the front you want to evoke). Possibility of focus : Roman Standards and Standard-bearers (Eli 221), for example...
Salve, Amice !