Hello Khaja,
For Case 1:
Rest protocol is not supported by Soap Adapter.So you should have rest adapter .configure it and give the endpoint to third party team.They can trigger the message to PO.
For Case 2:
Rest webservice is already there?Then again you need to have rest adapter.If you wish to make Http call and get a response go ahead with Soap adapter.
Below link can be referred for all rest scenarios