When talking about tank mates for Amphilophines, being cognizant, of water parameters can be an issue.
Amphilophines such as red devils prefer hard, high pH water.
Any cichlid (tetra or other fish) that comes from, and prefers low pH, soft water (if the RDs water is hard, and high pH, as it should be) will be under stress and at a distinct disadvantage.
Amazonian cichlids ike oscars that on the surface seem good tank mates (because of size), but prefer soft, low pH water, so may not be a great idea, or any other territorial cichlid for that matter, may be at a disadvantage, due to space concerns.
In a 300 to 500 gal tank, you may have some leeway, but in a veritable puddle, or rut in the road (the size of a 75 gal tanks water volume) your options, are severely limited.