The problem with questions like this is that there are absolutely no absolutes.
I think that Keyholes are ideal for this; they're a wonderful and underrated fish. I also bred Uarus for a number of years, back when they used to be referred to as the Poor Man's Discus, and they lived with a breeding colony of Guppies; I never saw an Uaru attempt to eat even the smallest newborn fry, and the little fish required ruthless culling on my part to keep them in check. Much more recently, I had 6 adult Gymnogeophagus balzanii living with two species of Goodeid livebearers (Ameca and Xenotoca) and while the largely-herbivorous Goodeids exacted a frightful toll on the cichlid fry when they became free-swimming, the reverse never seemed to happen.
But...a friend to whom I provided a "starter culture" of Uarus back in the day tried the Guppy thing and it failed miserably. Uarus are not efficient predators, but in the confines of just about any tank...and especially a tank with almost no plant life because the Uarus ate almost any type of plant...if they decide they want to eat Guppies, they will eventually catch them; he kept restocking his Guppies (usually with my extras!) and they kept disappearing. And a current neighbour who fell in love with the exotic big head on my dominant male balzanii, and to whom I gifted a subordinate male who wasn't "getting any" in my tank, reported angrily that he ate all her tetras within the first week.
And no, I did not utilize the all-too-common trick of overfeeding my fish to within an inch of their lives, hoping that obesity would keep them from choosing predation. That is
not good fish-keeping.
