I would get the rest out of their ASAP. African do not mix with centrals. RD's are HIGHLY aggressive so Im not suprised. Sorry to head about your losses.
Keep your red devil. They have great personalities and are so interactive. Ofcourse mine is moody so some days hes like a aquatic puppy dog, other days he's a big A-hole. He even bit my dog on the nose!
I went through the same thing as you a few years ago when I first got my RD for 5 bucks and put him in a community tank. The killings started within months. The good thing was it served as a good excuse to get more tanks as my wife is a softie and she had to save the other poor fish. Definately work that angle LOL.
My RD Pierre will tolerate very little in his tank now. When I put feeders in he often even kills the ones he wont eat. If they survive to see morning hes just saving them for later.
Once I move and get a much bigger tank I am going to try and acclimate him to a mate, but Im a little unsure of that.
Your joking right?
You don't want to crank this up a notch, you realize putting a Festae in a tank already owned by a Devil isn't going to end well. Salvini's are tough fish but will in all liklyhood end up dead as well.