I finally found some time to explore another RTOS, this time it's CoOS. My KL25 freedom board will be great board for this purpose.
I have played with their CoX library in the beginning, tested all demos and how their code looks like, but I wanted to move to RTOS anyway.
CoOS RTOS [polisoftdesign.com]
Some introduction (part1-part3) to CoOS RTOS.