What is the diameter of the posts? 1" or .75" ? (or other?) Do the wheels for heavy-duty shelves fit these posts?

The diameter for the posts is 1". The casters should fit into the bottom of the poles for this shelf.

Can you purchase additional shelves for this unit?

You can purchase additional chrome 36" x 14" shelves on this page - https://honeycando.com/collections/shelving-components/products/steel-shelf-800lb-chrome-14x36

For the shelving unit you are referencing the weight capacity for each shelf is 200-pounds. For these individual shelves you're purchasing, at that 36"x14"-size we only have the 800-pound capacity available but those will work.

Can the SHF-01606 3 tier 36”x36”x14” shelf come in black ?

We don't have those exact measurements in black.

The closest we have is this shelf:
https://honeycando.com/products/3-tier-black-14x24x30-250lb

And this media cart:
https://honeycando.com/products/3-shelf-tv-stand-and-portable-media-cart-black

How do you put it together no instructions 😠

I apologize there were no instructions.

You can find shelving assembly instructions on our YouTube channel here: https://www.youtube.com/watch?v=1gXI8y-8xC4