How do I change the drop down box options for Tracking Number on the Shipping Information Screen?

Hi,
I would like to either change the Shipping Method and Tracking Number drop down box to reflect the current Shipping methods chosen by that store OR remove the drop down box entirely.

I am using my own shipping plugin for Australia Post so how would I add that to the choices?

IF I deleted the whole drop down box but kept the field to enter the number would the tracking still record properly or would it not work? Would it default to 'Other'?

Thanks,
Nat.