Hi, as far as I know starting and stopping is a global action for the whole simulation. So stopping and starting a single robot is probably not possible.
I did not test yet, but I assume you should be able to start and stop the simulation also from a Python script and it should work the same for VC and KUKA.Sim.
I might have to correct myself, apparently i cannot remember my own postings. Apparently it is currently not possible to start/stop simulation from Python level.