I know you said you dont want small fish. But if you put something in with the firemouth that stays out swimming around like some swordtails or somthing of that nature it may make the firemouth feel more safe and it would venture out more.
You could do the texas alone in the 75g, might be able to get away with the convict in there too. I'd let them grow up together and keep an eye on them. As far as the firemouth, you could do a pair of them in the 40 gallon, but Its not enough room in my opinion to do a shoal of them. I'd recommend moving the convict if you get a pair of firemouths, they will probably want the whole tank to themselves.
I know you said you dont want small fish. But if you put something in with the firemouth that stays out swimming around like some swordtails or somthing of that nature it may make the firemouth feel more safe and it would venture out more.
I was thinking about some dithers however i would need to find the right dithers, id worry they would turn into lunch once the cichlids get a little bigger
Yeah with the firemouth. Not with the Texas. They can be skidish and hide a lot when they are alone. The dithers will send a signal to the firemouth that the cost is clear and there are no large predators around. I will give the firemouth confidence and make it come out into the open more.