If you give the units physics behavior, they will collide with each other and not overlap. Just set the world gravity to 0 else they fall off your layout. <img src="{SMILIES_PATH}/icon_e_wink.gif" alt=";)" title="Wink">
I believe Arima has done it.
Alternately, I've done it using Rex's SolidPushOut plugin: https://dl.dropboxusercontent.com/u/577 ... solid.html
Set your unit to that behavior and set them to Solid.
You will have to customize the obstacle to something else rather than Solid, trigger it via on startup etc.
This approach works too and uses very little CPU processing so its fine on mobiles. The only downside is they look like bump cars sometimes, they push out a bit too much. But honestly its just a minor issue, otherwise visually and gameplay wise, its fine.