Realizing a multitasking program with Timer Interrupts
Have I understood the concept of multitasking right?


there are different flavours of "multitasking", some more sophisticated than others. in a nutshell, it is about time sharing to provide the perception of concurrency.


  How can I allocate time slices for a "Task" or is it not possible in this case?


your approach to multitasking is less conventional. typically, it is a scheduler that switches in and out of tasks periodically. so the "time slice" is fixed.


  Can I replace the "Task" from toggling LED to ADC measurement or receiving bytes on USART? Or would that be realised differently?


that can be tricky. the key challenge is to interrupt a UART transmission in the middle of it and only to execute the same UART transmission in the ISR. not all microcontrollers are capable of that, and even if they are, it should be approached with caution.