Licensing Query for API usage


We are developing a custom app of “Twitter”. For development of a custom commercial usage app , please provide inputs on below mention queries:

  1.   Is there any Private API/ SDK available for Android & iPhone Platform ?
  2.   Is there an Licensing involved on using Public API for development of a Commercial App ?
  3.   In case, there is any licensing involved please let us know the steps to get the License & also provide us the other details too.