All FW flounders I know of are brakish water fish, might want to research that to be sure, but if it was BW that may explain why it did not eat. Some BW fish, due to stress, may not eat when in FW.
Either Bichir or Gar will be fine, my dad always comments on my tank and the harmony the fish exibit. The tank has 3 large Gars and 2 large Bichirs, all five fish are on par with each other as far as size goes, and that is the key to preditory fish, keep the size similar.