Can I stack these boxes on top of each other?

Yes, the design is built to stack. Line them up to maximize your storage footprint without spreading across the floor.

Will this handle a humid garage or outdoor storage?

Plastic construction resists rust and won't rot, so moisture isn't a concern like it would be with metal. It'll hold up fine in damp conditions.

What's the best way to organize small fasteners inside?

Interior layout is open, so you'll want to add drawer dividers or smaller containers for bolts, washers, and clips. That gives you control over what goes where.