I would like to slow movement speed down for all units but not necessarily reload speeds. It occurred to me I could maybe exploit extended units to increase reload speed but not movement speed and then set gamespeed to normal. I don't want players to have to mess gamespeed though. Is there a way to do what I'm describing, maybe an extended preplaced unit that slows movement exclusively?

IF you don't have too many units it might work for you to always move a burrowed unit under the unit you want slowed down (which is all units in your case). Very location and lag intense though.

Looks like I won't be needing this after all, jjf28 made a tool I can use for scaling up terrain (which accomplishes the same thing for me).