Are the casters locking or free-rolling?

The casters roll freely. Position the cabinet where you need it, then tuck it against a wall or bench when you're done working.

Can I stack drawers or remove them individually?

Each drawer slides in and out independently. You can empty one without touching the others.

What's the weight capacity per drawer?

Capacity isn't specified — treat each drawer as storage for hand tools, fasteners, and light parts. Don't load one drawer with heavy cast-iron components.