Pinkies go bad in the freezer sooner than larger mice. I've kept pinkies for 6 months with not problems, in six months your snake should be well beyond pinkies. I don't know if they go "bad" in that they are rotten and not good to eat. They do freezer burn or dry and a corn snake will stop eating food that's in bad condition like that. You could let your snake be the judge, they can smell if food is fresh or not. Freeze dried mice won't hurt a snake, but they probably don't have as good of nutrition and don't appeal to a discerning snake.
how long is you snake?
I a snake over 12-14 inches should be able to handle two pinkies no problem. How often you give two pinkies is more important. Offer it two, if it eats two then great, if not, it's not ready. At this point it's good to feed less frequently. Drop to a once every 5-7 feeding if you're feeding every 3-4 days on singles.