MMS and SMS are not considered data.  You are not charged to received SMS (text messages) if you have an unlimited text message plan.  Apparently, reading that link you are charged for MMS messages.

20 megs is pretty light for iPhone usage.  Remember voicemails come out of data on iPhones.  I signed up for a Free Google Voice application and forward my phone to it.  I get my voicemails as emails and make the decision whether to play them.  You can send and receive text messages out of data (albeit from a different phone number).  Google doesn't support MMS yet.