The flexibility of the legions came from more standardized equipment and extensive training, with extra emphasis on how to behave and respond to various situations. This allowed new recruits to more easily assimilate into the legion. i.e. Everyone knew what to teach them and the recruit quickly learned what to do, gaining confidence.
The strength of this fluid combination of cohesion and flexibility, was the redundancy that showed itself when a legion was faced with tactical setback and defeat, or when caught in a strategic bind. The training and confidence (moral) of the legionaries, enabled them to recover from a setback and regroup, rather than rout from a tactical defeat. They knew what to do, even if their immediate commander was killed. Contingency plans kept units from disintegrating and men from routing under duress.
Fighting in unfamiliar territory, I suspect that the legions were frequently caught off guard (under informed and/or underfed and overreaching), sustaining many minor defeats that they were able to recoil and recover from. Thanks to this redundancy, they survived to fight again.
A good analogy is that fighting a band of skilled warriors was like dealing with an egg. However tough, once you broke through the shell, it was all but over and they would run (pun intended). On the other hand, fighting a legion was like dealing with an onion. You'd have to peal away layer upon layer...
Caesar was far less likely to write about tactical defeats or strategic setbacks, unless they were too large to ignore when reporting back to Rome. He was also less likely to give credit to his Gallic/German auxiliaries, who were invaluable in reconnaissance, procuring supplies from the locals and fighting. If it lost its lines-of-communication, and more importantly, supply, a legion was vulnerable to disruption and defeat.
It's this level of legionary warfare that we are trying to recreate in Hegemony Rome. The level of unit diversity that we can include, will depend on what our small team of artists and programmers can put together after the main concepts have been addressed.
Anyway, we'll try. :)