I have a 9 1/2 inch Ornate and Palmas in the tank with at least 3 other bichirs no more than 5 1/2 inches each. You should have no problem unless your bichir is highly aggressive. Mine are cool so that's why it's no problem.
maybe yours is just overly aggressive? I've kept all sizes and varieties of smaller Bichirs with medium Laps and Endlis that were around 9-10". I too am surprised it didn't work out for you.