I had Mega Pirates two games ago, for the first time. When they came, I had already effectively taken an unbeatable lead against the surviving Torian, Arcean, Terran, Thalan and Krynn AI, and was armed with Black Hole Eruptors with Ultimate Invulnerability and Aereon Missile Defense as my defense options. I also had Eyes of the Universe. The Pirates had extreme Point Defense levels and Shield levels and were armed with Mass Drivers and Beam Weapons. I unified all my warships(small anti-transport interceptors, medium frigates and large battleships) into one parsec, fleeted as many as possible together into an Anti-Pirate Force, and rounded up all my utility ships into a stack under the Anti-Pirate Force for protection. I researched like wild into armor and mass drivers while my Anti-Pirate Force successfully managed to destroy 6 pirate fleets for the loss of one frigate and crippling of several others. By the time I had upgraded with Zero-Point Armor, the Pirates had backed off and concentrated against the AI races, so I went on the offensive and began slaughtering them, to the point that my surveyors could resume anomaly patrols under manual guidance. Then, while I had researched up to HD Spike Driver 4, all the Pirate Fleets vanished. Only the singleton ships remained, who soon got hunted down once my even more effective Black Hole Generator armed ships arrived. The AI races, with the exception of the Mass Driver armed Thalans, had been wiped clean of all their ships and starbases, paving the way for a constructor-spam session on my part which led to me taking over all but 1 of the galactic resources and setting up Influence Starbases that would soon conquer the galaxy(through cultural-assimilation-based conquest victory). The Mega Disease(not the Plague, mind you) event also occurred at the zenith of the Pirate Wars, but had no effect other than to delay my armor research by 2 turns.
Amazingly, the Pirates never attack Minor Races, who are able to keep their useless starbases and tons of trade routes alive through all odds. Maybe an extremely-mega Mega Event could be where the Jagged Knife appears with the Mega Pirates as their military force and declares war on everyone. Or an antithesis of this event where the Fundamentalists appear and are defended vehemently by the Peacekeepers. The first idea would have potential to conquer the galaxy, the second idea would cripple(or inhibit to an extent) all evil races in a galaxy.
BTW, is there any difference between the Pirates(who I've had in both the normal and Mega variants) and the Peacekeepers(who I still haven't seen)?