Garleans have been interested in and researching the Echo since long before Aulus's twisted experiments. They kidnapped Minfillia in the ARR MSQ precisely because she was an Echo bearer and they wanted to study it. Remember that a primary charter of the Garleans is to stop the Primal menace, and the Echo renders one immune to Tempering. That reason all by itself would have been enough to justify research into the Resonance. Zenos, naturally, came to the realization that it was so much more than just Primal-repellent.

As to whether non-Garleans manipulate Aether on a regular basis, to the point where it's regarded as a major advantage over the Garleans (albeit one the Garleans have more than sufficiently balanced with their Magitech), I'd say they do. Without their Magitech, a group of normal Garlean soldiers pitted against an equal number of normal non-Garlean soldiers would likely have the disadvantage - and back in the days when the Garleans were driven into Ilsabard, that was likely the situation in every encounter they faced. The development of tools capable of turning the tide is actually pretty admirable, or would be if not for the Garleans unending parade of atrocities they committed, both against their ancestral tormentors AND against peoples totally unrelated to their race's original oppression.