The depth of the bed can be about 6-12" (some even go a few inches deeper). Many gardeners like that extra depth for the plants to develop roots. But if you only want 6", that would be OK also for your garden. If the plants need more root space, they will try to go through to your native soil.

If your raised bed is already an established height, I would fill the bottom with a well draining substrate like sand and then top with your desired depth of growing medium. That way the sides will not shade the growing plants while they are still small, and air circulation will be better. My raised beds are 18" and this is how I have set them up. I like the height so I can sit down and make weeding easier. Protection from predators may still require a short fence. I have had rabbits climb into my unfenced beds.

ditto on the sides shading and the air.

I have new 20" deep 4x8' raised beds. I was planning to fill in the bottom with sand, then put Mel's Mix at the top. My question is how far from the top to fill the soil, noting concerns above to prevent the sides from shading the seedlings. I was planning to fill to 1" below the top. Is this ok?

I would put Mel's mix at the top since it will do the plants the most good there. Tomatoes like a 12" depth but most plants just feed on the top 6 inches of the bed.