Units can't automatically stack. Use turn off vision AI scripts or pre-place stacked buildings. There is no other way.
Units weren't suppose to be stacked on top of eacht other. If you do stack them then they will spread all over the place! The reason why your keep crashing is cause there is too much movement from the units that were stacked.
You can stack burrowed units in the editor but thats it. Normal units just move all around. And you can stack with triggers, but its almost a waste since you'd have to almost perfectly set up the situation for it to be useful.