I'm trying to get creep colonies to spawn around a creep colony that's already there in a plus sign (so immediately to the left, right, up, or down, no diagonals). It'll move them alright to the left and upwards, but they don't go the the right or downwards. I'm using overlords for mobile grids, I suspect that if I use a unit that's just ever so slightly larger, it might work.

http://www.staredit.net/wiki/Grid_Systems

Large air units like Overlord and Devourer are only 1.5, perhaps try using a 1x1 (scourges and observers alternated) grid extended out enough to get where you need.

Thanks! I figured it out with a little experimentation:

An overlord, then a scourge was too much. Many scourges was just a pain in the butt. A two stage grid setup worked the best. For moving down or to the right, I changed from using overlords to using queens. I then used 8 scourges (remove 7) to go to the right, and 3 scourges (remove 2) to go downwards (I may have mixed those up).