Some variety is allways nice!
While prototyping with a single model is fine, the final game should offer some variety in enemy behavior and visuals.
So I got myself the Tiny Toon Zombie pack from the unity assetstore and was absolutely amazed by it.
Not only that those are the cutest zombies that I have ever seen.The package contained much more outfits and exchangeable heads than shown in it's preview pictures. 
On top of that everything is on a single Atlas texture!
So it only causes me some slight bumps in performance, compared to a single model.