I've pirated. Once you have your fleets set on piracy they automatically try to attack merchant fleets. Its really easier when the player has no escorts and have merchants stacked up large groups, the highest I've seen is 200! Anyway, the pirates capture the enemy merchant ships and add them to your pirate fleet along with whatever goods they have in them. You'll have to bring the fleet to a nearby planet to trade the goods, and dismantle the fleet to get rid of the merchants. This is another reason I'd like to be able to manage the ships in a fleet to be able to divide the merchants up from the subspacers.
You can still manage to snap up merchants even if they have escorts in the same fleet, but the chances seem to be pretty low. They can fight back, and I've lost a few pirate fleets from running them into large patrols of cruisers and destroyers.
