Restrict Access to Make Edits in Process Requisition

Summary:

Once our Requisitions have been approved we are looking to see if we can lock down fields within the "Process Requisition" task so that the user cannot edit those fields. All the approvals occur at the Requisition level, and if the person who can "Process Requisitions" can edit fields then it negates all the approvals acquired at the Requisition level. We are looking to reduce the amount of fields within the "Process Requisition" area under the "Add to Document" action.

Separately - is there a way to have the negotiation flag checked and have the style be anything else besides the standard purchase order on auto-created purchase orders?