The explanation that I've heard indicates that the electrical impulse range of each fish forms a 'bubble' (imaginary zone, kind of like personal space) around the fish. When things are within the bubble, the fish detects them to some extent--as living critters, plants, rocks, whatever. Two fish, each within their own bubble, will conflict if the bubbles overlap one another frequently. This overlap disrupts each fishes ability to interpret their own impulse patterns, resulting in aggression and stress.
So, in theory, if the tank is big enough for each fish to maintain the integrity of their 'bubble', you shoudn't have many problems. The problem with this theory is that I haven't seen anything that indicates how big each bubble is for each fish. Some fish might have a rather large range--say 2-3 foot from their body.