I am also a T-Mobile customer and I have to say that all of the instant messenger programs you mention are web based programs. Based on this fact, you should not be charged for any messaging that you do through them. What you will be charged for are MMS and SMS messages which are entirely different from instant messages.
It has been my experience that several of the T-Mobile reps I have spoken to seem rather ill-equipped to answer our questions about services, coverage, etc. Every so often, I will come across one who is masterful in their knowledge of their company services as well as what their devices can do. However, I feel that many listen for keywords (such as "messaging" or "billing") and then give the answer they know related to those keywords. Unfortunately, you will not always get the correct answer.
Have you visited
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