GREAT EXPECTATIONS. Overjoyed at the ouster of tyrants Manuel Noriega and Nicolae Ceausescu, Miami's Cuban community is giddy with anticipation of the end of Fidel Castro's rule. Proclaims an electronic billboard: NOW MANUEL NEXT FIDEL. Calmer heads are also prepared for what is no longer unthinkable: the Dade County police have drafted a crowd-control plan for the inevitable street celebrations that would erupt if Castro fell, and the Immigration and Naturalization Service is gearing up for an influx of free Cubans.