The biggest problem I have always had with space pirates is: Where the hell did they get their military vessels?
If they are attacking and seizing trade vessels, then I would expect to see an ad hoc squadron of converted ex-merchants (and possibly a small escort or two).
But if they have a shipyard churning out corvettes and destroyers (or larger), then how was this shipyard built, and where are they getting the resources to build more ships?
If space pirates have the resources, manpower and infrastructure to be building and maintaining military fleets, then they aren't pirates - They have achieved nation status.
They might be a predatory nation, such as the Barbary Coast states, but they are a nation nonetheless.