I once added (exactly) 10 yellow lab fry and 23 acei fry to my main tank of full-grown mbuna(Largest was daddy acei 6" long) and a 5" eyebiter. The fry were about 1/2~3/4 inches long, and not a single one got molested! In fact, the fry grew better with the increased tank size and heavier filtration!
Although I may have had some luck with my fish indeed....The same acei as above didn't even touch 2cm long duboisi fry! Man I shoulda just kept them!!
Oh, sorry....I guess IME I second GTZ....just large enough not to fit into the adult's mouth(But my eyebiter....well he was an oddball :lol: )