Can we order another separate garment rack to put under the second shelf? (Just the separate rod part to actually hang clothes on)

Yes, this rod can be added to make dual hanging bars. https://honeycando.com/collections/all-closet/products/adjustable-hanging-closet-rod-chrome

What is the load capacity of this rack?

The weight capacity is 25-pounds.

Can we purchase extra shelving for this unit?

Yes, here is the shelf you would purchase for the garment rack.

https://honeycando.com/collections/shelving-components/products/steel-shelf-350lb-chrome-18x36