By today's standards what was done in the 70's with a 5 gallons was like 12 cories, as much harlequin and 3 powered dwarf gouramies. It was a job to keep on top of. And there was a lots of action in there.
Misconception sprinkled with ignorance made me do stuff like that. Where the fish mistreated that bad... I don't think so... While none ever reproduced for obvious reasons. They lived too long.
Or a tank completely over run with guppies, that looks like a school yard at recreation time. all the time.
What can make a fish happier in life than sparing with one of his buddies and a little pump of adrenaline. As long as there an escape place and you are there to intervene if things get sour.
Endlers... Will overrun any tank anyway. Let nature take course. They are promiscuous loving critters and feel at home when the only thing they can see and feel are, Endler's as far as they can.
So the question is. Assuming being able to maintain good water quality and lovable environment.
Who is happier, the tank with 6 lost or the one with a hundred shoaling in front of the water output ?