Business rules are attached to objects - not forms. The system evaluates all rules for an object whenever the object is created or changed. Condition 'IS NEW' is TRUE only when an object is created. Condition 'WAS CHANGED' is TRUE when an object/attribute is actually changed, which can also happen during object creation, for example when initializing an attribute (the original value was UNDEFINED).
It does not matter which forms are used to create or change an object. Objects can be created by rules without using any forms at all. You need to specify appropriate conditions for each object rule if you want to control when the rule's actions are executed.