Invoice number/sequence for freelancers

As a freelancer earning less than 20L therefore exempt from GST registration, I have two types of invoices that I generate:

1. Invoices to domestic clients
   
   I follow a simple template to generate these in INR. These invoices have my PAN.
    
2. Invoices to clients outside India
   
   These are typically generated from PayPal (may be generated from Payoneer as well in the near future) in foreign currency (USD/GBP,etc.). These invoices usually just have my name and email and do not explicitly mention my PAN, though PAN is required to open a PayPal/Payoneer account as per Indian regulations.

Though I am exempt from GST registration, I wish to follow the invoice sequence rules that are under the GST, as it will help maintain consistency when I get registered hopefully in a year or two. 

I have the following questions about invoice numbers:

1. Can the two types of invoices above have different series (considering they have different formats) or should they belong to the same series?
   
   For example, if my PayPal USD invoices start from 17001, the next PayPal invoice is 17002, and so on. Should my third invoice which is a invoice for domestic clients in INR be 17003 or should it be a different series considering it looks different from the PayPal ones?
    
2. Does the invoice sequence have to reset every financial year or does it continue?
   In FY 2018-19 should I begin from 18001 or continue the 17000 series from the previous financial year?

Any suggestions and answers to my questions will be greatly appreciated. Thanks.