I've considered pirates for a long time but giving them an economic rationale that fits within the game is tricky. It would be difficult for them to establish a base and create all the support needed (fuel, minerals, etc.) and it would relatively easy to find and destroy them due to jump point choke points.
However, with my recent focus on WH40k I have been giving some thought to a spoiler race along the lines of the Dark Eldar. In WH40k, they use the 'Webway', which is an ancient network that exists outside real space, and live in a vast city that exists in the depths of the Webway. They raid real space to satisfy their lust for slaughter and to gather slaves.
In Aurora terms, that could be an alien race that lives in a different dimension on the far side of the Aether from our own dimension. They are capable of transiting through the Aether to raid our space. The rationale for that could be plunder, slaves, glory, etc. The benefit of this type of approach is that it address the twin problems of basing and choke points. They are based on the far side of the Aether and they can enter our dimension at different locations without using jump points. It still needs more thought, such as can they raid anywhere, or are there gateways that can be found and eliminated. Also, how quickly can they depart our dimension? Too fast and they would leave when attacked, so there needs to be some limitation on that ability.
In WH40k, the Dark Eldar ships are fast, stealthy and hard to target, but relatively weak in terms of armour and shields. They also use boarding tactics. That general style can be replicated in Aurora, using cloaks, high ECM, etc. with boosted engines and minimal armour. The intention would be to present a threat against commercial traffic, surveyors, etc. but not something that would threaten a major battle fleet. The 'pirates' would be primarily commerce raiders that would require outposts, patrols and escorts rather than fleet engagements.