Okay, 6 attempts later and I seem to have it down.
However, how would one make the debris invisible to sensors, and how would one make
Vasudan debris, as it all seems to be Terran regardless of what I set it to in the editor.
Moreover, perhaps we should mark that particular function of the editor as 'Asteroid/Debris Editor', to save people like me from having to go to the trouble of experimenting in alternate ways for an hour and half, and posting topics on HLP...

Edit: Scratch that, i've worked this out. Setting it to create Terran debris creates Asteroids, setting it to create Vasudan debris creates Terran Debris, and setting it to create Shivan debris creates [extremely ugly] Vasudan debris. Damn, someone
really needs to clean this editor up... But my main question remains, how do I make the debris invisible to sensors or simply untargetable?