If you have more than one race group, master scheduling (Pro Version) will simply alternate which race group is currently racing.  This keeps all race groups involved, as they don not have to wait until other race groups finish racing before they get their chance.  Racers still only race others within their race group.  Master Scheduling does take into account group size.  So, a smaller race group will run fewer heats than a larger group before the schedule alternates to a different race group.