Do I need any additional hardware to mount this to my hitch?

The step bar mounts directly into your 2" receiver opening. Confirm your receiver size before ordering — if you have a 1.25" hitch, this won't fit. Check your hitch specs or measure the opening to be sure.

Will this step bar rattle or move around once installed?

A 2" receiver fit is snug by design. If you notice movement after install, tighten the pin or bolt securing it into the receiver. Most rattle comes from loose fasteners, not the bar itself.

What's the weight capacity, and is it safe for standing on while loading?

Weight capacity is not specified in the product data. Before loading heavy gear, test-stand on it yourself and feel for any flex or movement. If it feels solid under your weight, it's ready for use — if it flexes, contact support before relying on it.