Gst returns for new registrants

Two queries on GST:

1. For new voluntary registrants - when should they start filing GST returns. a) From the next month of registration (i.e. if they got registered in August, are they liablie to file returns for August as well) or b) from the next month of raising first bill after registration. 

2. There is an exemption for expenses upto 5000 per day for reverse charge. If one is paying commercial rent of 20000 per month. Can he be exempt from reverse charges.