Not only are frontosas not compatible with the other 2 neotropical cichlids, but the red devil might end up a solitary "wet pet" in that size of tank if you think you can place an oscar or frontosas in there long-term.
The oscar is more aggressive than the frontosa not to mention the different water conditions. The oscar will terrorize the frontosa just like your red devil terrorized your oscar. How big is the oscar by the way?