Yes they can Mix, stop listening to these people who have never done it. I have 10 large fronts with all types of fish and with my stingrays, never an issue at all.
Your fronts happen to be in a 600 gallon tank with a lot of different fish and a colony of fronts. We're talking about 90 gallons here with 5 single specimen, two of which get 15" in length, that's a gigantic difference. In a less than ideal tank size, blackbelts can become one of the meanest sobs in the world and there's no way that a Front in that small of a tank can stand up to it. No matter how many times you ask this question, the answer won't change unless you suddenly end up getting a much larger tank.
