The setup should last for a good long time before possibly needing to move them to the 75. Firemouth are not know to be fast growing. There are reports of males getting 10 to 12 inches but that's years down the road.
How Firemouth work, the males will split the tank into territories. The females travel around in a pack checking out each male till they find one to breed with when they are ready. So having more is better as it spreads any aggression to more fish. Firemouth are more show than actually fighting. Lots of gill flaring, some back and forth pushing. Thier mouths are rather small so not much actual biting.
As far as numbers. I would anywhere from 6 to 10. Same with the livebearers. The livebearers will reproduce like crazy, some will get picked off by the Firemouth, but make sure there is some plants for the fry to hide in. A surface plant like water lettuce and or some Vals will help the livebearers fry make it to adulthood.
This tank is just a 40b but I started with just 7 Vals and 6 Plattys.
It had a Finnex planted plus 24/7 light, and got some direct sunlight late afternoon's. No ferts used, no CO2 just sunlight and an army of plattys and convicts feeding them thier waste. I did use Caribsea planted plus substrate, so that come with some ferts, no other feets added.


This is what I started with.
At 6 months I had to pukk out all the non Val plants, remove half the vals and replanted. I had some Vals that were 6 foot in length.
