Have objects ignore collisions with some other objects?

I have a bunch of objects spawning and going up and down the map, problem is I would like them to not collide with each other at all and get stuck together. It only becomes a problem when I have all my unit types spawning, because there may be 10 or more on screen at a time. Any ideas on this, I would like them to just move through each other, and not collide...

I thought Layers might do this, but the docs say its really only used for raycasting and selective rendering...