The principle is true for African cichlids but could not be further from the truth w Central American cichlids
I wouldn't make that generalization. Many CA cichlids are social, or at the very least gregarious to some degree, and stereotypes of aggression are often the result of the whole "tiger barb syndrome" thing we're all familiar with, where social species, when kept alone or in too low of numbers, take that as an opportunity to take over a larger expanse of territory given they no longer need to share (and are subsequently more aggressive). Thorichthys are naturally shoaling yet have a reputation for being aggressive in aquariums. When kept in groups, they exhibit spawning synchrony, and will often collectively rear fry with neighboring pairs. Amatitlania don't exactly exhibit spawning synchrony but will spawn at the same time almost coincidentally, based on a shared interest in spawning sites given conditions are right. Certain Amphilophus species in the wild (mainly benthic species) are known for spawning in loose colonies, and idly shoaling. Rheoheros and Cryptoheros are known for the same. Cribroheros and a lot of other rheophilic species are naturally shoaling. In my experience, cramming does decrease aggression, as I can have 3 spawning pairs of Amatitlania in a 3 foot tank with no injuries or casualities, plus the pairs will keep their fry in tight balls in their respective portions of the tank. If I were to say, have a single pair + an extra in the same size tank, the extra would quickly be picked off. I have a pair in a 3 foot tank alone right now that let their fry spread out and feed across a third of it. If I had a shoal of tetras in there (which I have before) they would keep the fry low and close.
I too have collected CA cichlids in the wild and will be doing so again in a few weeks. Not all CA cichlids inhabit large rivers only shared by 1 or 2 other cichlid species, or maybe none at all. Niche partitioning plays a big role in coexistence. In Panama, where Duane lives, there's only really small picking detritivores/insectivores. However there are a few ecosystems where there are many species of cichlid living in close proximity. In tributaries of the Rio San Juan in CR, where I've collected, tiny streams can host over a dozen species of cichlid at the same time.

This stream was only about 6 feet wide and 2 feet deep at its deepest. Most of it was only about a foot deep or less. However, in it I caught:
Amatitlania siquia
Amatitlania septemfasciata
Neetroplus nematopus
Hypsophrys nicaraguensis
Parachromis friedrichsthalii
Parachromis dovii
Cribroheros alfari





Jeff Rapps also recently collected an equally small stream a few miles south of this one.
In it he caught:
Amatitlania siquia
Amatitlania septemfasciata
Cribroheros rostratus
Cribroheros alfari
Parachromis dovii
Parachromis friedrichsthalii
Neetroplus nematopus
Andinoacara coeruleopunctatus
He additionally relayed to me personally that Tomocichla tuba were present as well.
This proximity is possible through niche partitioning. The two Amatitlania are able to coexist because septemfasciata prefers faster water and has a higher protein diet. Two Cribroheros because rostratus is more of a benthic sifting feeder vs alfari from what I've seen being more of an opportunistic omnivore. They loved hotdogs in my experience. People catch them on dry flies as well. I'm not sure on the two Parachromis, often 3 because managuensis is present in that river system as well, however I assume size of prey is likely a factor as lacustrine populations of dovii eat a lot of Amatitlania and Neetroplus. I can imagine friedrichsthalii eating more insects and smaller fish like livebearers or tetras. Herotilapia multispinosa can also be found in these little streams alongside everything else. Cribroheros longimanus and Archocentrus centrarchus are allegedly present as well. That brings us to 13 species of cichlid in one river, with many often being found in close physical proximity.
Lake Nicaragua proper has a good variety of species, namely:
Parachromis dovii
Parachromis managuensis
Cribroheros longimanus
Cribroheros rostratus
Neetroplus nematopus
Hypsophrys nicaraguensis
Herotilapia multispinosa
Archocentrus centrarchus
Amphilophus citrinellum
Amphilophus labiatus
and possibly a few other undescribed Amphilophus sp.
Rio Terraba with:
Amatitlania sajica
Amphilophus lyonsi
Parachromis friedrichsthalii
Cribroheros diquis
Cribroheros altifrons
Andinoacara coeruleopunctatus
Rio Sarstun with:
Cryptoheros chetumalensis
Rocio spinosissima
Rocio octofasciata
Thorichthys helleri
Thorichthys aureus
Mayaheros urophthalmus
Petenia splendida
Vieja melanura
Theraps irregularis
Chuco godmanni
Trichromis salvini
you can argue that spinosissima is only found in the little backwater streams but that still leaves us with 10 species of cichlid in the same river.
The list goes on. Just wanted to get my foot in the door there.
Anyway that said I wouldn't put more than a single Herichthys in less than a 6 foot tank, given you want Herichthys. Obviously they get big, and the ones that don't are usually too aggressive (or expensive) to have that many in such a relatively small tank.