For cichlids it is more about the "how" we aquarists do things, than any other factor, except water quality.
If you buy 8 (or whatever number) juvies of ? cichlids, and put them in a tank all at once, and not add any more, the chance of success is very high (at least until they pair up) even if the tank is a bit small.
But if you buy one, put it in a tank, then another a week or even a few days later, so on and so on, your chance of success lessens exponentially because of the territorial nature of cichlids with each new purchase.
The larger the tank, the more wiggle room, but in tanks of less than 6 ft, options are very limited.