It depends on what the cichlids are ment to do I guess. I bought 4 convicts to breed for food and ended up with 3 females and 1 male in my 20 gal tank. Two of them hitched up within the week and then were nearly killing the other females. I put in a tank devider, the fish beat themselves up attacking the devider. I put cloth by the screen, one of the females got ick. I got fed up after that and didn't want to infect the whole tank so the two unpaired fish got chucked into the piranha tank beat up and sick.
I didn't see them for 3 days so I assumed that they were in fish heaven. I managed to find them the hard way, middle of a water change I'm watching the p's making sure everyones behaving and something small darts out. I scream, jerk my hand and hit popeye in the side and nearly fall off my chair. Lucky enough the fish, chair and myself were all ok. Odd enough too was the fact that the ick was completely gone.
The convicts are 2 inches, the 5 piranha are 5-8 inches. Nearly three weeks later there still alive and now the convicts are confortable enough that they have started moving rocks (only the red rocks not the black ones mind you) and fight over territory betwen themselves. The smaller one even trys to school with the piranha when they get scared and dart off but shes too slow.
I'd say it can work, I'm looking for a male or two to throw in the big tank to hopefully eliminate the breeding tank altogether possibly. But at the same time if it doesnt work it's a $3 fish. I would definantly make sure the fish you want to live is twice the size of the one you don't care about as much.
Btw though, its kinda morbidly gross when one of the cichlids decides that it wants to eat the talapia with the big fish
Pardon the cell camera, theres one of my monster cichlids in the skull.