Collecting rare candies to use in bulk on your starter is a really bad idea (screw you child me) for a variety of reasons. Normally when your pokemon fights they gain hidden levels dubbed "EVs" that correspond to the 6 stat types and are decided by the pokemon defeated. Using rarecandies will level up the pokemon, which then adds the regular base stat increase but ignores any EVs stored, meaning that a typhlosion trained solely in battle vs an identical one given ~43 candies to boost it up would be stronger.

Also it says candy in the name, obviously eating a bunch of those will fatten your ass faster than you can say dracozolt (which we need more art of as well).