[Suicide Junkie]

The way it works is:
a) The race has no colonization ability
b) The race is given bonus components for improving their ability to capture ships cheaply.
Small boarding party components, anti-boarding defense crew toxins, shield disruptors, etc.
c) The race is given bonus components for living in space... Tiny spaceyards, no maintenance costs, completely undetectable ships (Blend in with the civilian traffic), etc.

All combined, this gives you a race that will quickly spread out and build small bases and outPosts in secret places. They will be able to use their stealthiness to pick and choose the targets they attack (unescorted transports, small colonies, etc). They can then plunder the planets and deconstruct the ships for cash and tech.
Low or No maintenance allows them to survive even after taking losses to unexpected resistance, and when the other races have their military doing extra patrols and escorts.