1/ To avoid damage to both your bikes and your vehicle make sure the down-weight limit of the bar is more than the combined weight of the bikes and rack.
2/ There is no warranty if an Interchangeable/Convertaball type tow ball are used because they have too much movement in the ball head. The neck of this type of system is also often the wrong shape for the conical clamp to attach to.
Supplementary Plates are for when your equipment obscures your vehicle's number plate and can be attached to your bike carrier etc. Available online at www.licensys.com for approx $22