To me, it all pivots on what's behind that tile. If it's the same brick that the fireplace is made of, then I don't see a problem leaving small gaps between the tiles.
Also, to my way of thinking, for that fireplace to remain functional, you're going to have to set the tiles with a thin set that's meant for the high temperatures you'd see in a fireplace.
I think your best bet is to bring this question to the Tile Contractor's Association of America:
Tile Contractors' Association of America
I'd contact them directly or any of their members who manufacture thin sets to see if you need a special thin set for tiling a fireplace.