Except that using hired guns allow you to avoid the militarization of the society - Britain was able to limit its standing army by doing so for centuries.
Carthage might not be peaceful at all, but it aggression was nowhere near Rome or Alexander the Great.
Prolifically employing mercenaries and being a militarized society are not opposites. Macedon was very fond of mercenaries and also a thoroughly militarized society.
Carthage was more less aggressive than Rome and Macedon because they had a way lower manpower pool. At the height of the empire, there were ~500,000 Phoenicians within its borders. Ruling over ten times that number of Greeks, Berbers, Iberians, Gauls and Italics. Even with their mercenaries, Carthage was not able to raise the kind of army that would have been necessary to establish dominance over the Mediterranean. That isn't to say they didn't try though - they tried really, really hard, their military just wasn't that great.
On the subject of the Dutch Republic, trying to expand would have been demented. Anyway they turned, they would run into a bigger and stronger neighbor. France, the HRE, Spain. This was the main motivation for being quiet and peaceful on the European theater - in the colonies they were quite aggressive.
I just don't think that merchant states are inherently more peaceful than big agrarian states. Note that the steppe empires, from the Xiongnu to the Timurids, could all be classified as mercantile states. The primary goal of their expansion was the securing of important trade cities.