An RTOS is anything that allows you to divide your application into seperate tasks, and gives you some means of scheduling those tasks and communicating between them in a safe way.

That does not necessarily mean that it is pre-emptive. Cooperative RTOS's also exist, and are easier to implement than preemptive ones. That's because you don't have to consider the cosequences of a task switch happening when you don't expect it.

The thing that is not usually worthwhile on small platforms is plugging in someone else's code that you don't understand.

It all comes down to 'waht do you want to do'. If you need to send data over a serial connection, but you can't afford to wait while each byte is sent, you already have a need for some kind of 'serial out' task which is interrupt driven, and which shares memory (an output buffer) with the rest of the app. Some kind of simple OS is needed.

As for the 'RT' part of RTOS, well ... all systems are Real Time to some extent. In as much as waiting for ever for a result is never acceptable.

It all comes down to defining and understanding the requirements.