I totally agree with KF. When putting together an aggressive cichlid tank (American or African) if possible stock them as juveniles, at the same time. As noted by KF, tank dividers might work for adult fish. Another possibility is a very congested hardscape, with numerous line of sight breaks, hiding spots and backdoor escape routes.
I'm a little confused "not difficult to do" I read that you stocked a Midas and then a Midevil?
If so, the Midas and Red Devil / Midas hybrid are probably too closely related to expect a peaceful cohabitation.
It's important to note that starting a group of aggressive cichlids together as juveniles isn't always successful. That method totally failed with my Texas Cichlid Hybrid. Many Central American cichlids lose their minds when they hit puberty and become psycho killers. Such is true with my fish. He is now confined to his own private tank. I'll never know what combination of cichlids were crossed to create my Texas Hybrid, but judging by its shape, Midas comes to mind.