I said privateers and I meant privateers, not "Yah me hearties" Hollywood pirates. The governments of Europe routinely used to issue letters of marque to individuals for a percentage of thier take and yes, while dedicated warships are expensive, converted freighters are not.
To my mind, it's like a pirate that's managed to lay his hands on a Perry Class frigate - it ain't gonna happen. Too many safeguards, not to mention guards.
Okay, I admit I don't see it like that. For a full size battlewagon, yes. But since Prof. Cavour made his first trip in what was basically a big steel ball, that's not the case for smaller vessels. Anything airtight will do. So small ships, possibly made from scrapped vessels, or built by some of the up and coming private corporations would work. Just slightly larger than the space fighters perhaps.
The system already allows sloops or similar sized vessels, so why not? Frieghters are not protected, the Spanish are recreating the halcyon day of looting South America, it would make sense for some government to allow a few small vessels, using 'pop guns' and boarders to go grab some of it while having deniability.
As to the Selenites being unusable, certainly they can be used as sepoy infantry in the wars to conquer the moon. As to crew for the ships? They're not a stupid race, they can make tools and thus can be taught. Not much on these ships is amazingly technical, it's coal or oil fired tech. And ships always need painters, stokers, stewards etc.
I mention Standard Island as you had included Nemo and Robar but seemed to have let that one slip. There's a floating city somewhere down there guys.