I have a 150a rated incomming service. I want to install a genready loadcenter that is only available with a 200 a main breaker installed. Do I have to use the 200a main or can I install a 125a double pole breaker in the branch breaker section and backfeed the bus and not use the 200a main? The existing panel is being done this way with a 60a double pole. But it is a main lug panel.

You really should use a MLO panel. The 200 amp does not need to be used, however, if you remove it you will void the UL Listing of the unit, and easy gotcha for inspectors and insurance adjusters.