I feel like I'm beating a dead horse (you probably feel that way too, Bryon, sorry about this) but I've read that having more than one species of cichlid (as long as they are regionally compatible) promotes more natural behavior in the individual fish and causes them to be more active. I don't want to stress my fish out, but I feel that being aware of territories (your own and your tankmates) and interacting with other fish that would be encountered in the wild is a naturally occurring part of a fish's life (granted, in a MUCH bigger space in the wild).
There are a few "generalities" here that I must counter so others are not misled. And let me start by saying that "experiments" with fish can sometimes work, but I follow the expected norm for the species, not the abnormal experiments. We don't always know why individual fish within a given species may not behave normally in a given situation, but putting fish at risk is not my idea of responsible fishkeeping.

This:
having more than one species of cichlid (as long as they are regionally compatible) promotes more natural behavior in the individual fish and causes them to be more active.
is not true, nor wise, and probably stems from a misunderstanding. With African rift lake cichlids it is a different "kettle of fish," but deliberately introducing stress to a fish in an aquarium is not good. Most Central American cichlids never see another species of cichlid in their habitats. Providing a representative habitat for any species to introduce natural behaviours is fine; introducing fish that are foreign is not the same thing. The stress from the former is normal and the fish species can handle it, provided the space is sufficient to provide the proper environment. Stress from the latter is abnormal, and going to weaken the fish. This is scientific fact, not supposition.

I'm just hoping that the combination of lots of caves/broken line of sight/etc., small school of distracting dither fish, and 3-times the filtration could allow me to keep a Honduran and Firemouth in a 40 gallon tank. They will inherit a 70 gallon in 2 to 3 years.
Filtration has not the slightest impact on this species interaction that we are considering here. Tank space obviously would; in a truly large aquarium, this could work; though even in public aquaria it is not attempted.

Fish release pheromones and allomones and these can cause considerable stress. Filtration does not remove these, only water changes can.

Byron.