i'm not sure if i fully understand the question. you want an Instant Messenger service that can use your SMS count as opposed to the typical usage of data?
If so, you can sign up for a free AIM account (aol.com) if you don't have one already and configure it to forward the messages to your phone as an SMS when you are offline. I have this with my iPhone... i get a text to my phone from the persons screen name and i can reply to it. however i just sign on to AIM and reply that way because i have unlimited data, but you don't have to. This should work for you.
Also, why don't you have unlimited data? are you on T-mobile?