Anders J,
Selecting the most appropriate RTOS depends a lot on the specifics of your application and your priorities (e.g., are you willing to spend $).
A BSP for the EVB9S12XEP100 EVB is available for free download at the Micrium site. It isn't much trouble adapting the BSP to other S12X models. I've done it.
Good Luck,
-- Jim Ledin