In a species like the Dominator Invincibalis, a zombie infection would have to spread really, REALLY fast, because they kill each other frequently, even if they're not zombies.
Maybe we could add a simple "enemy number" system, where, if it's only one zombie against two of the species it's trying to infect, it would run away. If it was only one zombie against one of the species, it would circle rapidly around it to prevent getting attacked. And if it's a horde against a few of the species, just overwhelm them!