Well, first of all, I'd like to state that I have no primary accounts or formal education on the matter of Roman martial arts, but I do have a acknowledged amount of knowledge in HEMA, or Historical European Martial Arts, so take my post with a pinch of salt if you will.

First of all, I'd like to address the manner of the gladius. Contrary to what I believe you've been hearing, the gladius was not necessarily the "brute force, all-in" weapon that is traditionally depicted. In fact, it was a weapon used with finesse and skill (in the idea that you're stabbing someone, of course) that matched any other weapon of its time. I'd like to point out that due to it's average length (avg. 20-24 inches, averaging around or below the length of contemporary hellenistic swords) and its design, it's not a weapon to be used as a direct-force weapon. It ironically lacks the average shortness and designed to be used as a sort of brute "piercing" weapon (such as a rondel dagger) while it also lacks the length and design to be a piercing weapon based on reach and leverage. However, its design allows it to excel in its own little field.

Secondly, I'd like address the actual use of the gladius. From what I've seen, interpreted, and speculated, the gladius was specifically designed to exploit both human anatomy and contemporary armors. You would not be attacking the chest of an opponent, but rather parts of the body that wouldn't be armored. When fighting an average or even heavily armored soldier such as a Republican Roman Legionary, the main targets would be the thighs, groin, and then (skipping the chest) the neck.

On the note of cataphracts: I think this matter is both related to martial arts (and the use of Roman weapons against heavily armored cavalry) and the mechanics of cavalry charges. I'm forced to keep this brief due to my own time restraints (will expand if I can), but when Romans faced cavalry, they did not rely only on their swords. They had pila, which contrary to popular perception, that could be used effectively as thrusting spears (albeit awkward ones) and hastae, or spears, which could be distributed amongst the legionaries receiving the cavalry charge. On the other hand, the main opponent of cavalry was not pikes, spears, or guns; but the mass of the infantry. If the infantry could present a significant wall of men, then both the horses will be reluctant to charge against what they perceive is a solid formation and the riders will have a harder time essentially "breaking" through the formation (whether that be physically or psychologically.)

Sorry if my post is short and choppy, but do reply if you have further inquiries. I'll do my best to reply.

EDIT1: Additional reading on effectiveness of maille armor (And basically a lesson on why you can't stab through most armors) http://www.myarmoury.com/feature_mail.html