I would also agree that the size tank (40 breeder) is way too small for anything other than the smallest cichlids. If that's what you used for others in the past, its not surprising there were aggression issues.
My choice for that size tank would also be Cryptoheros nanoluteus as far as Central Americans go.

male above female below

they are right size for the tank, beautiful, non-aggressive, and available from places like Wet Spot
My choice for that size tank would also be Cryptoheros nanoluteus as far as Central Americans go.

male above female below

they are right size for the tank, beautiful, non-aggressive, and available from places like Wet Spot