The photo shows a black latch with gold anodized latch release. The color is listed as silver. Which is accurate? Well, the photo here looks silve

I'd never seen that black picture before. They're silver-colored, just like all the other photos shown of them.

How much clearance do I need to leave between my gate and the post?

Most gates leave enough space for hardware in their dimension. A 16' gate will actually be about 3-4 inches shorter. The opening should be 16'. This latch needs about 3"-4" of clearance between the post and the gate.

Does this come with all of the needed parts, or is there a gate latch bolt kit you also need to order?

You will need the hex screws to bolt the latch assy to the 6x6 post. Everything else comes with the kit. These latch kits are bullet proof. Don't cheese out and buy the cheapochi-nee knockoff junk for $2 less. Pure junk. Co-line is what you want.

Want to latch 4 foot gates (2 inch tubing) to 4x4 wood posts. Will this latch fit, or be too big?

The back plate, that goes against the post is 4-1/4 inches wide, so it would fit and extend slightly on each side. The mounting lag screws are centered width-wise and 1-1/2 inch apart, so that works. The main problem is the latch is made to fit 1-1/2 inch tubing. You could probably get a welding shop to replace the part that goes onto the tubing, at additional expense. It could also be opened up and reformed to fit 2 inches, but the mounting holes would not be in the right place - another problem. It's a nice latch for 1-1/2 inch tubing.