I think you understood your 2º a bit wrong.
It is not primarily talking about the transaction ME21N and ME22N (just used it to differentiate create from change), it is talking about the form of the output (see the F1 field help), whether you print the whole PO or just the changes.
If you never had any message for the old PO types, and now you created a message determination and these old POs match with your new setup, then it is unavoidable that you get an output.
You would need to have the PO creation date in the access sequence and your conditions, then SAP could know if the condition is valid for the old PO too.
Maybe you can add this as a requirement to your message schema, then you get only an output if the requirement is fulfilled.