I'm trying to make it so that all buildings on my map allow units to pass through them, but cannot be placed on top of one another.

I tried every variation of "Footprint 2x2 "Footprint 2v2 (Placement only)" "Footprint 2x2 (contour) and no footprint at all in both "movement-pathing footprint" and "Movement - Pathing footprint - placement" and none of them worked. Either the buildings could be placed down fine, and units still couldn't walk through them, or the buildings were able to be built on top of each other. I only saw them placed on each other in the editor. I don't know how it would be if a unit built them in the game (haven't gotten to that point yet). I also tried messing around with the collision. All buildings have "Movement - Collide" set to nothing.

If there isn't a way to do from the buildings, is it possible to do this on the units? I wouldn't want the units to have collision with each other though (a radius of .4-1 seems to work depending on size). I tried putting unit inner radius to 0 and that didn't work either.

Edit the buildings Movement - Collision to have only structures checked. Edit both footprints of the building and remove everything on the pathing layer (for me this just requires editing it and hitting ok right away since it does not seem to load the pathing information)

Quote from Temp

Edit the buildings Movement - Collision to have only structures checked. Edit both footprints of the building and remove everything on the pathing layer (for me this just requires editing it and hitting ok right away since it does not seem to load the pathing information)

I'm not sure what you mean by remove everything on the pathing layer. I set Movement - Pathing Footprint to what and Movement - Pathing Footprint - Placement to what?

Edit the footprint itself. Edit the pathing layer of the footprint so it is blank (set to None in every cell).

Quote from Temp

Edit the footprint itself. Edit the pathing layer of the footprint so it is blank (set to None in every cell).

Sorry, I guess I wasn't clear before. I don't know what you mean by the words "pathing layer". Both "movement - pathing footprint" and "Movement - pathing footprint - placement" have the word pathing in it...but I assume "pathing layer" means something else...i just have no idea what.

Setting both "- pathing footprint" and "- pathing footprint - placement" to nothing (with buildings having structure collision) makes it so buildings have no connection to the grid and they can be placed on top of each other.

I don't know if this will help you much or not. If you can open the Engine of Destruction (first level with the odin) level in the editor, then you might be able to see how it works there:
http://wiki.teamliquid.net/starcraft2/Campaign/Engine_of_Destruction

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Cheap tactic (works on brutal): You can block off Odin with buildings, but ONLY BEFORE he leaves a destroyed base. For example, after Tychus destroys the first base, he'll walk downwards for a refreshing drink. If you wall Odin in with buildings before he exits to "drink", he'll get stuck there and won't move. However, if you let Tychus walk out of the enemy base, and then try to wall him in while he's drinking, it won't work because Odin will just walk right through your buildings later. If you do wall him off, the game will eventually give Tychus the ability to walk through buildings, so him being stuck won't last forever.

I assume it's activated by a trigger in the mission...but there are hundreds to look through with not very helpful titles >_<. I hope there is an easy way to do this in the editor with the buildings. Thanks though.

Actually, if you look at the other part of the description, it's probably more useful:
"However, if you let Tychus walk out of the enemy base, and then try to wall him in while he's drinking, it won't work because Odin will just walk right through your buildings later."

It seems that Odin probably has the ability to walk through buildings most of the time, but the ability is disabled when he is in 'base attack' mode so that he doesn't walk through enemy buildings?

Maybe comparing odin unit stats in this level, and the Media Blitz level, you could spot obvious differences?

Setting "movement-pathing footprint" and "Movement - Pathing footprint - placement" to "Footprint 2x2 (underground)" made it work.