You can soft reset for Xerneas, but it won't ever be shiny unless you hack. You can still soft reset for IVs though.
You can soft reset for a shiny starter, but the chance of getting a shiny starter is 1/4096 for Gen VI. There's no way to increase the chances of getting a shiny starter without hacking/ tampering with the game while soft resetting. It's probably better and less time consuming to finish the game and then use the Masuda method to breed for a shiny that you want.
Source: experience, Bulbapedia - Shiny Pokemon