This pastry cream will work fine for millefeuille. The pastry cream must be chilled before placing it on the pastry; if piped on warm or hot, the pastry will become soggy. Chill it and then whisk it until smooth before piping.
You can make the dessert a bit a head of time, so it chills properly, but keep in mind, the longer it is stored in the refrigerator, the softer the pastry will get. For the best results, I would assemble it and eat it within the same day.
In terms of the icing, you can try following this procedure for
how to make fondant. Hope this helps!