I agree NOT to use a RTOS. I always make a switch / case construct running as a tick (high priority interrupt) driven by a timer ranging from 50..500us depending on the processor. In this "switch case 0..x" I let the background program provide ADC, mean values, output of synchronised IO, communication issues etc. This makes the main loop in most cases quite simple. It will do floatoperations, PID regulations triggered by bits in the tick etc.
If necessary, I trigger a low priority interrupt within the tick, whick handles slower tasks.
This gives me control over the whole timing issues etc and makes the system react in realtime.

Werner