Changing collision behaviour

I have a number of units (for waves, if you will, for a map of mine) that are used in both a hero siege and a td (minigames).

For the hero siege, I require them to have a collision size (preferrably 16), because there are places in the map that you're supposed to be able to fall back to and only get hit by a few enemies at a time (if they're melee).

For the td, I want the creeps to be able to pass each other, as slow towers otherwise tend to get too good. The obvious solution would be to use different units, but considering how dull it is making waves, and that the enemy units work fairly well for both balance-wise, I prefer using the same units.

I cannot turn off collision detection, since there are parts of the TD that one can build mazes in. Therefore, I need to emulate either smaller collision size (0?) or a wind-walk style ability. The problem with using a very long wind walk with an equally long fading time (at least I think it is) is that the creeps will not attack towers if the path's been blocked.

So, what I want to achieve here, is having creeps with essentially different collision sizes depending on the situation.

Thoughts? Suggestions?

Isn't there a "Turn collision off for (picked unit)" action? I might be wrong, but I can check in a few hours.

There is, but I said I cannot do that. The problem is they'll walk straight through the towers, which they mustn't. Only each other :)

EDIT: Hmm, it seems I was wrong. It appears giving them wind walk with a very long fade out time actually works ok (although not perfect). I believe this is because creeps will be able to attack until they're entirely invisible, but I'm not sure. I'd still appreciate a better solution, though.

Wow, sorry, can't believe I didn't read that.

I think the only way (Which you already said) is with a spell based off of invisiblitity or windwalk. You could add and use a windwalk based ability whenver a unit is hit by a slowing tower/has slow casted on it (I don't know of any better solution).

If anyone knows a better way, feel free to share.

Windwalk with a -1 transition time makes them never go invisible but retain the WW pathing.
Try that, might be what you want.