I just kegged my chocolate milk stout, half of which is destined to become chocolate mint milk stout with the addition of extract.
I started testing the mint flavor by adding 1/4 tsp to a quart, and it was WAY too much. I'm cutting it down to about 1/4 tsp per gallon instead (1/4 for half a gallon is still a little too minty). Turns out it is really easy to overdo it. Too much mint really destroys the drinkability of the beer, so tread lightly.
I wouldn't use candies because extract would be easier to control. Store bought extract should only contain peppermint oil and alcohol, which is the best you could hope for by steeping leaves in vodka, but it will be much more consistent if you ever decide to do it again.