1) randomTrips.py can generate vehicles with fixed routes (--route-file) as well as vehicle trips that only specify origin and destination (--output-trip-file). If you use the former, vehicles will pick the fastest route assuming an empty network. If you use the latter, vehicles will use the fastest route considering the state of the network at the time of departure.
2) randomTrips.py accepts the --vehicle-class option to generate vehicles with that specified class. Just run randomTrips multiple times with different options. Make sure to also set the --prefix option to avoid name clashes. I Advise to use osmWebWizard.py to generate a scenarion with different vehicle classes and use the generated files (including batch files that call randomTrips) as a reference:
http://sumo.dlr.de/wiki/Tutorials/OSMWebWizard3) Calibrators do not spread the inserted traffic evenly to allow for vehicles to arrive later during the calibration interval. To avoid this effect you need to define multiple calibrators that cover shorter time intervals
regards,
Jakob