Maybe I've been starring at the floor too long, but the only way I can think of doing it is with a fake unit and a death OCL. Get an invisible unit (non-selectable), give it a lifetime min/max of 1500 (should be enough time to get into position) and a death OCL of whatever units you want. It means they will just appear infront of the building instead of walking out. Or you could do it angried mob style with a nexus... that sucks know.
Oh... I have another idea. Dont know how or if it could be done, but you could make them have like a hoard bonus thing that works over long range, and the units cheap. With the bonus they fire at a normal rate, but without then they fire and move really slowly, making it impossible to use them well on their own...
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