Current setup is: One main stage object--A (with GUI from FXML) and buttons. Clicking buttons on the main stage object will create objects from another class --B. B objects have different GUI FXML. In this case, is it best to create B objects each with its unique controller? Or do drive them by one controller?
I suggest to divide your GUI into some logical custom controls using the fx:root construct.
1 FXML -> 1 root/ controller
Here's the tutorial: http://docs.oracle.com/javafx/2/fxml_get_started/custom_control.htm#BABDAAHE