Are you looking to have multiple windows? Then I would use your first code in your first post with some modifications. However, your question is about working with multiple classes. It is possible but you need to be aware how to access the main sketch and only access it if you need to. For instance, if you have a class which is design to sort your data, then this class does not need to have access to the PApplet. Instead, you can pass data to the PApplet. You can do this by instantiating the object within the main sketch and then passing a reference of the PApplet to it as describe by @cumulus in the previous post. However, the most effective way is to return the data to the sketch so it can handle it. What strategy to implement depends on what you want to do and it is the fun part of the designing stage.
Kf