Hi Rahul
For outbound IDoc interfaces, what you plan to do will not work (assigning IDoc as message type in new service interfaces.) The runtime will always resolve the IDoc reaching PI into the standard IDoc imported definition - it will never use any custom service interfaces.
Yes, you will and should have different partner profiles, but of what partner types - LS, KU, LI?? If you do not have this information, please ask your ABAP/IDoc team. Without this it is hard to determine whether you can switch from classical to ICO for new interfaces. And if not, then it might be best for you to just stick with the existing design/practice.
Rgds
Eng Swee