Hi,
I have a friend who is a retired caterer. She said reheated is never quite as good as fresh, though it is what you can do.
If they are to be reheated, she prefers to keep the latkes on the small side, because they are easier to heat through. She recommended draining them very thoroughly with paper towels. When you are ready to reheat, then preheat the oven to 350 degrees. Put the latkes on one layer on a cookie sheet. No layering. No foil on top. Reheat them until they begin to sizzle, then serve immediately.
Are you planning to simply boil your pierogis or are you pan frying, too? If boiling, then you can precook them at home and drop them in boiling water for a minute or so to reheat. If you are panfrying, then maybe take the same approach as the latkes.
If you have time in advance, you may want to experiment on reheating the pierogis to be on the safe side.
Let us know how it all worked out.
Regards,