A random rule of thumb ratio I use, is each adult cichlid needs 10 gallons of water volume per inch.
This is "not" using only its length, but adding together L + W + H.
So as an example ....
a single 14" inch Vieja, that is 10" tall, and almost 4" wide needs minimum of about 240 gallons.
add a 15"L managuense that is 6" tall and 4" wide, the tank needs another 250 gallons.
Of course none of these calculation take in the instinctual territoriality that either cichlid might display as maturity sets in in a mixed community, and all Central American cichlids display different degrees of territoriality (aggressiveness or going wacko over night (as is often posted here), is sometimes used erroneously substituted for territoriality when it sets in) at different stages of life .
And as exolucius mentioned, water quality would be a major player in the scenario.
Although filtration plays a somewhat partial roll, most filtration doesn't take care of nitrate (water changes are practically the only solution).
I consider the efficacy of most commercial filters to be vastly over rated.
And consider nitrate as simply an indicator of other deleterious substances in the water.
Nitrate is not just the bad single entity, but we don't have the ability to test for all the others.
So for whatever amount and size of large cichlids kept, in whatever volume of water, its the management of nitrate and its nasty allies that controls the population density.
I like to keep nitrate on or below 5ppm, to avoid HITH disease, and the other health issues, we see mysteriously pop up in the disease section.
