Hi Jayati,
Please find my answers to your queries:
1. I think you should use the data model 0G as it is the latest data model provided by SAP in its EHP5 and EHP 6 versions of MDG. for better understanding you can see the entity types from both the data models and can understand that the 0G contains much more detailed financial entity types than 0F.
2. if you decide to use the data model 0G, then there are more than 10 entities you can create. Few headers are Account COA, Account Company code, Cost Center, Cost Element, Profit center, Its Groups and hiearchies, also Reposrting structure items can be created.
3. Try activating your data model first, generate structure and then try using this activation method, it will work fine. It will bring up all the pre delivered edition types and change requests for you to begin your processing.
4. yes it is absolutely upon you and the scenario you are trying to develop when you select predefined workflow template. You can find the details of these workflows in sap online help http://help.sap.com/mdg61 . you eventually can use the rule based workflow as well or in some cases advanced workflows.
thanks & regards
Rahul