appointment plus timing settings to 15 mins

1. The default time for any appointment in appointment plus settings is 30 mins....i want some appointment to be of 15 mins. How can that be achieved. Is there a option in the settings or code. Very important as i had brought the plugin with quick 15 mins consultancy agenda and it is not there now. I am stuck.

2. I want my service providers to do all the appointment setting through front end...how can that be achieved.