If you are going to charge for the features, you should not make them free, even to begin with. By doing so, you create an initial value in the users' minds of what those features are worth... $0. If anything, charge for the Pro features and gain a good user base (though you might not sell a single Pro account), and then have a one-day special where users can sign up for a month of Pro for free (or something along those lines). This allows them to get a taste of the Pro without eroding the perceived value.
You might also consider making the whole thing free and asking for donations to help you keep the project going and continue improving it and adding features.
As far as building a sustainable business goes, Twitter can't even do that from their own service. It will be difficult at best for you.