As someone who's worked with the current pirate AI in terms of building something to be used by the pirates that scales against the ship sizes of the players, All I have to say is: The entire concept presented is flawed in one major aspect.
No one person will ever use the same classification system as another, down to the letter. It is this fact that gives us the "God given right to make up whatever the hell we want"and call it a day.
Now, while I agree that the current scaling is broken as well, I cannot agree to any of the suggestions posted here because they are just as broken as the current system, albeit in different ways.
Whooplaah's Method: While flexible, it's ultimate flaw is that players have the right to call any ship they build whatever they want and get away with it. I have seen battleships and titans that people have built exceed 100,000 mass and even 1,000,000 mass, and at the same time, I've seen people class ships with 500 mass in the same category.
Crimson-Arist's Method: the flaw in this system is non-uniformity of the pirates at varied sizes. You aren't going to find SSP and SMP servers that run the exact same pirates as each other, and you sure as hell aren't going to find multiple SMP servers using the same pirates either.
Case in point folks. Let the people build what they want and let them fight what they want. We have a sandbox game that is still in ALPHA STAGE DEVELOPMENT that will, without a doubt and without skipping a beat, constantly update. And one of the systems that WILL EVENTUAL UPDATE is the current AI Mechanics and with it, the Pirate Mechanics.