I've been wondering about collisions between different instances of the same sprite! The family approach sounds great.
But in this case, I tried the approach using the families, and it mostly worked, but crashed sometimes and I couldn't figure out why.
Then I tried a horrible approach that *didn't* use families, just to see if it was possible. This worked without crashing, but sometimes created enormous beasts due to inherent flaws in the method.
Collision between the dust bunny sprites was always unreliable: sometimes it'd work, sometimes it'd ignore the collision. This happened in all circumstances: family or sprite collision, 'collision' or 'overlapping'. I'm still puzzled about that one.
I might go try this in a minimal setting, to see if I can reproduce the crashes (or better yet, create a working solution). Here's the cap, if anyone's curious.
http://dl.getdropbox.com/u/906550/assist.cap
PS: Great sunlight!