I haven’t made them with any other type of milk if you regularly substitute that type of milk in baking then I’d guess it would be ok.

Yes, you can frost the cupcakes a day ahead. You can put them in the fridge so they firm up a bit which makes them easier to handle before transporting and if your kitchen is hot from baking it won’t affect the consistency of the frosting. Allow them to come to room temperature for an hour before serving.(which is often the right timing to get them out of fridge, into a box and transport and place on table and serve) That will ensure you don’t mess the frosting up, they will taste fresh and delicious and are easier to transport then room temperature frosted cupcakes.