I so want to run your mod but since I am running custom troops and equipment I don't think it would play well with my current mods. Got you tagged for use for my next run through though.
I agree. Roman accomplished what it accomplished because its armies were armies comprised of professional soldiers who trained and fought as an army for years at a time very much like the standing armies we have today. I think an enlistment period in the Roman army was like 10 years or something like that if I recall. During that time they trained constantly and developed some of the most effective tactics ever devised using very complex formations such as the famous testudo formation but they had many others some even encompassing the entire army and designed to draw in and break up an enemies army. However that didn't magically prevent them from making mistakes or letting their guard down.
That battle your referencing I think is The Battle of the Teutoburg Forest. The Roman commander let his army get totally strung out across miles completely overextended and exhausted. From what I have read, the trail the Romans were strung out on was no more than 60 feet wide so there wasn't even room for individual centuries to get into a fighting formation much less the entire army and the German forces took the army completely by surprise, just rushing them out of the woods at close range ended up overwhelming the Roman Legions. I don't think the Germans actually used unit tactics though, it was more of a bum rush into scattered and disorganize units with no cohesive command and control. Basically the Commander made a huge mistake and paid for it with his and his soldiers lives. Actual unit level tactics on both sides, good or bad had nothing to do with it. The best I think you could say is that that Arminius had utilize a great strategy to lead the Roman armies into an ambush and thus disadvantaged by their positions, exhaustion and complete surprise, were wiped out by a relatively disorganized mob of German warriors.
Also historians all agree that in an open field battle, Roman could not be defeated by the Germans and in fact they never defeated a Roman army in a field battle to my knowledge either before or after this colossal mistake by an inept Roman commander.