Another option if going with a negative Endurance - they can move faster / through obstacles if they wish but doing doing so requires an Endurance test against the obstacle / speed with any shifts they don't make being taken as stress & causing consequences.
It gives you the picture of a zombie dragging itself through a broken window to get at you...and leaving a trail of its own entrails in its wake. Or the zombie who makes a sudden lunge across the room to grab you...and you hear its own bones snapping as it does.
Without much (or any) points in a dodging skill, they're going to be taking plenty stress and consequences as is, they don't need to be falling apart on their own. At least, not the fresh ones.
As for rules, instead of stress, give them a threshold you need to overcome. If you don't cause 3 stress in one hit, you do surface damage that isn't even worth tracking. You either do enough to take the thing out, or you do nothing. Of course, that's past whatever armour rating you decide to give them from toughness.
I've mentioned why I'm against the "take it out, or do nothing" approach before: Consequences, I feel, are a good and convenient way to model the fact they can take a lot of punishment but keep coming--just without an arm or leg by the end.
For a horde of zombies rolling athletics for all of them will be a chore anyway.
I use a bot for rolling purposes, so it's not much of a big deal to roll a dozen times in quick succession.
I'm sorry, maybe I'm just misunderstanding the rules. If you spend your action for the round moving, and without taking into account obstacles and blocks, you should ALWAYS move 1 zone at minimum. You can move 1 zone as a supplemental action and then do something else, even! You should not move backwards or fall over simply because you have a Mediocre Athletics and chose to roll that skill that round. That's dumb.
Well, mostly I was going to use the rolls to determine which, if any, zombies are actually getting into range of the PCs. A conflict might start with the PCs at one end of a hallway, and a couple dozen zombies at the other, say, two or three zones away, and I'd roll a handful at a time to see how much of the horde is managing to get close. I figure giving them Average Athletics would end up with more making the roll per turn, cranking up the tension for the PCs better.
In all, I think I'm going to go with 0 Athletics and a stunt to boost it when pursuing prey. That plus Zombie Toughness ought to make them easy to hit, but still able to pursue and able to take a lot of punishment before they're Taken Out.
How's this as a revision:
Romero Style ZombieHigh Concept: Shambling Flesh Eating Corpse
Other Aspects: Driven By Hunger; "Removing the Head or Destroying the Brain"; Slow but Steady; Never Just One Zombie
Skills:
Great (+4): Might
Good (+3): Fists
Fair (+2): Alertness,
Average (+1): Contacts
Stunts:
No Pain, No Gain: Extra Mild physical consequence
Predator's Drive: Athletics is rolled at +2 when pursuing prey.
Powers:
Living Dead [-1]
Claws [-3] (Venomous, biting)
Zombie Toughness [-3] (2 extra Mild consequences, 1 extra Moderate)
The Catch [+2] (Headshots)
Zombie Moan [-1] When spotting prey, a zombie may make a Contacts roll at +2 to summon more of their friends. This allows any zombie within earshot to take advantage of Predator's Drive.
Total Refresh Cost:
-6
Stress:
Mental OO
Physical OO
Social OO
Notes: The classic shambling zombie is slow to move, and on the surface looks fragile, but as a being driven solely by hunger, simple things like wounds and lost limbs aren't going to make it stop. Zombie Toughness and No Pain, No Gain mean that while they have no effective dodge score, they can take a lot of punishment before they go down--they should take every consequence except Extreme before being truly Taken Out--and should get back up again anyway unless someone puts a bullet through its brain. One zombie shouldn't be much trouble for a competent combatant, but they can still be dangerous if they get close, with a Weapon:2 venomous bite rolled from Good (+3), and Great (+4) death grip (har har) for grappling. If that wasn't bad enough, any zombie who spots you might use Zombie Moan to bring more to the party, and any character Taken Out via zombie bite will rise as one within the hour. Use the time it takes them to shamble into range to put them down, or at least blow enough bits off them to make good your escape.
Older, more decrepit zombies might have Athletics, Might, or Endurance one point lower--and thus no physical stress track at all--and come with consequences already filled. Fresher or just tougher zombies might have a point in Endurance or Athletics, and the stunt Tough Stuff to add armor against blunt objects.