The way I see this is:
1. Pirates usually, rarely, have the cash to blow on buying custom made, special purpose ships. Instead, they use prizes, cheap hulls that are modified, and the like.
2. They make up for lack of cutting-edge custom ships by using more of them. That is, they don't go pirating in a single ship.
3. As a rule, they don't engage in space battles. Instead, they use reconnaissance, snitches, and the like to find out when their target is in a position that makes them helpless and defenseless. For example, they wait for the target ship to set down on a poorly defended world and begin refueling or something.
4. They use overwhelming force and intimidation to minimize own casualties and swiftly take their prize.
I would think most piracy, short of having the pirates onboard the target ship, would mostly operate planet-side. They'd be more like Viking raiders than pirates trying to take a ship in space.