Gst & export of services.

Hi, i have three queries.

1. My online website business makes a turnover (via online advertising alone at 100% - and over 80-90% revenue is collected via bank transfers) of 15 to 25 lacks a year (FY 2015-16 it was within 15 lacks). We are registered under Service tax since we make over 10 lacks. Are we applicable to pay GST at 18% or should we wait to see if we cross 20 lac threshold this year and then pay?
2. We have users all over the globe and we use google DFP to place an ad from an overseas client who pays from abroad in USD to target global users. Should tax under GST of 18% be paid since we are a pvt ltd company based in Delhi or should we refrain assuming this as export of services since the service involves global users and the payment is from overseas and in USD?
3. Are we applicable under compounding levy? If yes, what are the pros&cons.

Thanks.