From my long-term terrifying biome fort- mobility is the key. Five 3 or 4 man groups stationed throughout the town is the safest bet. That way, you can respond to any zombies quickly. Legendary+ troops have pretty much no problem killing any amount of zombies IF there are at least two of them. Otherwise, the zombies will knock the soldier down for 20 minutes till one gets a lucky hit. Don't use bladed weapons, you will just multiply the problem. I honestly would not recommend armoring citizens, the loss of speed is very dangerous and armor doesn't seem to do crap against zombie blows. You state that ammo is in short supply- i'd recommend outfitting your important dwarves with a crossbow and a few bolts just to give themselves a last chance if the military can't get there in time. You can butcher any non-sentient corpses and then make bolts from the bones. Either having a large, very highly skilled military, or having a wall with a drawbridge and a door is important. If you just have a door, a stupid kobold zombie or a snatcher will let everyone in. Compartmentalization is very important. EVERY space needs to be able to be sealed instantly by you; this mitigates zombies breaking past defenses. Having several routes to every important area and having multiple stockpiles, sleeping areas, dining halls, is all crucial. That way, if your military is slaughtered and half the fort is overrun, it's entirely possible to lock all the doors to the overrun area, use the other food stockpiles, setup or use other farms, engrave slabs to the dead dwarves, and rebuild a military.
About the military- imo no point even trying to fight the zombies with a large but unskilled military. Zombies seem absurdly fond of headshots and they will utterly wreck an unskilled military.