Invoice Serial problems

Hi Folks

We have quite a big problem with payplans related to invoice numbering.

We are using [[year]].MF.[[number]] in the invoice serial counter configuration which is working well except

1) How do you RESET the counter from 1 again? We are up to over 200 now due to rigorous testing, and the client wants invoices to start from 1 for launch.
2) We are recreating a lot of user accounts, and giving them a partial subscription for free. This is automatically generating an invoice.. even though it is only for 0 Euro, this is (again) incrementing the invoice counter and giving the appearance of gaps. We do not want zero invoices.. only those with a figure

Is it possible to ONLY generate an invoice serial if the amount is greater than 0, where is the code that injects the invoice serial so I can potentially put an IF statement in there and skip invoice serial generation.. leaving that field blank AND not incrementing the numner

Thanks in advance for a swift reply