is it necessary to have a cooling tray with fan? Aren't perforated sheet pans acceptable?

MaDe – it depends on what you are trying to cool … and what are you trying to cool it in, and how long you have to wait.

In general, the latent heat of crystallization (the heat that is given off when chocolate cools down) can be carried off more efficiently if there is some sort of airflow. Even with a perforated sheet pan, having a fan to improve air circulation, especially on the underside of molds, is a good idea.