Construct2 uses a "pick" method to determine what instance to operate on at any given time. If you do not "pick" an instance it will apply the action to all. This is what you are experiencing.
You "pick" an instance by referencing it in the conditions portion of an event.
Sounds like you simply need to add a "is-not-moving" condition to the event that sets up your turret actions. You can do this via an instance variable in the object itself.
Add a .capx of what you got so far if that doesn't help you any.