You can make them however you like. Being a GM has its advantages. The GWM errata is available for free online, so that might help.

Quills is a core mutation, as are claws, wings, and scales, as well as large and small. So you can use those to 'reskin' mortals or creatures and create 'basic' zerg. You won't have mechanics for spawning new critters, creep, and the like. Area attacks are harder to get, look at firewands and improvise, perhaps? Or helltech.