Hi lIm…
I agree with Alistu Sir that you can change the indivudual bit rate locally in PRBS generators.
I am sure that the information you are requesting can be found in OptiSystem manuals. The example of Global data rate usage is shown in Lesson 6 (if I am not mistaken) online. You can also check the optsystem documents for the information about global parameter window and PRBS generators.
For your reference i am attaching a link which might be of some help.