I have approximately 56 chickens right now, and a bunch of up n coming chicks, and a couple dozen ducks. There are many many feral cats, feral dogs, coyotes, etc out here...and my neighbors think its "fun" to tame raccoons, squirrels, and possums enough that they aren't scared of people or dogs, so we have a bunch of those critters around constantly, too. And yes, squirrels will kill chicks. They don't eat em, they just kill em and take the food.
To ward off this impending disaster, I use 1" chicken wire for everything. I cover all ventilation holes and cracks, fence repairs, and line the tops and bottoms of my poultry areas with the stuff. I have to use a push broom instead of a rake when I do my cleaning, but not a single loss due to predators in four years (so far). I got the idea from an old-timer who has been keeping chickens since they were invented, and he has no problems either... he even deals with some kind of wild mountain cat with his flocks.
The only thing I worry about is snakes. I haven't yet had trouble with those, though. I imagine hardwire cloth would take care of that problem if it is an issue for you.