This has been discussed and debated here for years. I myself have read lots of arguments for and against, and I've tried it both ways. There is no hard and fast rule on this, and everyone's experience with it is different. Success or failure depends on lots of different factors.
I think it all boils down to personal preference and personal style of fishkeeping. I, personally, don't like the look and feel of a tank that is stocked too heavily. But, that means I have to be more disciplined about not buying too many different types of fish.
That was harder to do 2 years ago because I'd not ever had all these different species before. Now, I've got it narrowed down to a few species I really want to keep, and I can do without the rest.
I think the if you're gonna try it, you need to do it with Vieja and a few Amph's--maybe some Severum. I don't see too many people trying the overstocking trick with Parachromis. Can't do it with Umbees and Dovii.
I think a bigger tank is always better, too.
Be ready to do lots of water changes, and lots of vacuuming.
And keep in mind, a fish that's not too aggressive at 6-8 inches will change his mind completely at 10-12 inches. If your friend has 30 4 inch fish and they don't fight too much, I'm not surprised. If they're Vieja, they will when they're 12 inches.