I can see the need for less powerful zombies, such as small animals, etc. With this system, it would be very easy to customize. All the GM would need to do is reduce the number of Stress tiers by one or two. For example, a zombie rabbit might only have 1 tier of Stress instead of 3. This would make it much easier to kill with one shot.

Also, some zombies might already have Stress by the time the PC's encounter them.

I think having 1 hit - 1 kill zombies makes good sense.

While I usually let the Players know which enemies are the minions, for the genre, I could see not making it clear.