New concrete reaches full strength after 21 days but it is recommended to wait the full curing period of 28 days before installing tile.
No. The colour of your tiles are oven-heated at temperatures exceeding 2300 degrees. As such, they are impervious to fading.
High-gloss products will be more slippery than low-gloss products.
As long as the existing tile is cleaned thoroughly and sound, and the right adhesive is used, this should be fine.
Yes, tiles for outdoor areas have been specifically designed for that purpose. Their design features cover issues such as slip resistance, water absorption and changes in temperature.
Any tile can be used on the face of a fireplace. Putting the tile directly in the firebox is not recommended, but high-fired ceramic tile is often used directly on top of and surrounding the firebox.