2. Can I put it on existing tiles? What surface can be applied?

If your tiles have smooth surfaces, you can apply Tic Tac Tiles over existing tiles, including the grout lines.
* A partial list of ideal surfaces for Tic Tac Tile includes: smooth ceramic tiled splashback, smooth glass tiles splashback, wall panel (acrylic), wall linoleum, mirror, wallpaper.
* Partial list of dusty and porous surfaces that are NOT recommend for tiles: stucco wall, stone wall, unfinished wall (gypsum or plaster), unpainted plywood.