Hi ,
1. Do we need to consider ROS in a separate client?
No , Its not required to use a separate client incase of ROS.Its only recommended in case of SUS.
2. How do we define the questionnaire ? Any step by step document available to do the same.
For questionnaire .SPRO->SAP SRM -> SUPPLIER REGISTRATION -> BASIC SETTING -> CREATE/CHANGE QUESTIONNAIRE. Here you can define your own questionnaire by going into the survey and defining your own survey or can use the standard one .Then open it in change mode .Assign questionnaire (if you have created your own then assign your own questionnaire)or can use the standard one.
3. What are the configuration steps involved for this?
Regarding configuration . The above step will lead you to the configuration part for questionnaire. For details please do have look to the below link where you can find all your doubts.
SAP Community Network Wiki - Supplier Relationship Management - SAP SRM
Cheers
Gaurav Gautam