Is this possible when using dbca to create the rac database and instances?I don't know if the instance name is set when using dbca as I have always used the manual method but the database name is set at that time and I think instance name is as well using your naming convention. The instance name can always be changed afterwards. You set this when you export the ORACLE_SID and start the database. The instance name can be anything you want and does not need to relate to your database.
Also, is it a good idea to have the instance names different from the database name.Your naming convention is typically what is used as there is no confusion. All my database and instance names follow the same convention. ie [database_name][instance_number]