Flooring Tile - Thinset or Mastic?

I am applying 16 inch, ½ inch travertine over a zero clearance fireplace. The area surrounding the fireplace was drywall when I ripped out the old tile, so I replaced it with the same. In addition, I will have a ledge above the fireplace that will also have bottom/face and top tiled as well. This shelf protrudes about 2 inches from the wall.

In any event, I am tossed on what to use for the tile. The compound the original contractor used was mastic, though the tile was ceramic. I am told by the tile store that I should use durabond flex thinset, though using thinset on drywall to me seems wrong (I could be wrong) In addition, I was told by other parties that since my travertine is crème colored, I should use white thinset as not to discolor the tile.

Also, about 4 inches of this tile will overlay the fireplace. The surface of the fireplace is metal. I could tell that the original contractor used some form of adhesive from a tube and it was strong, but uncertain what it was, or if it even was correct for that matter. My friendly Home Depot representative stated I should use firestop sealant, which to me is a "sealant" and not an adhesive. So, he was obviously incorrect, though I still am uncertain what to use. In theory, the zero clearance fireplace metal area is not supposed to get hot, though with this directly above the FP, I am certain it gets warm. To me, liquid nails would work well, but I am unsure that it would handle the heat.

Any comments would be great!

THIN SET TAKES HEAT VERY WELL.--ITS BASICLY CEMENT AND LIKE SOME WISE CONTRACTOR SAID THEY USE IT TO HOLD BRICKS TOGETHER IN FIRE PLACES. (yea you john)

Who you talkin' 'bout, Eric? :D