If they said everything was fine it wasn't overstocking that killed your fish. Also if your fish were still young they probably didn't produce enough bio load to stress your tank out. As to stocking in a 55 you could probably have managed if you got rid of the flounder ,"most get BIG", When you say a school of coral cats how many do you mean? Also,if you want a reef a copper band isn't the best fish to have either.
hth
Max
hth
Max