If I read the initial post correctly, you are asking about adding a pair of angelfish solely to eat livebearer fry. This is not at all good.
First, angelfish are fish in their own right, and they need a suitable environment. No mention is made of tank size, but they will attain six inches body length with a vertical fin span of 8+ inches, and this means a spacious tank. Second, the fish must bond; a male will not accept any female, and vice versa. If you only get two, they must be a bonded pair or one of them will likely be dead before long. And as others have already mentioned, some of the other fish in this tank are not suitable for angels.
Second, they will not be able to adequately control livebearer fry. And no fish should ever be purchased to deal with a "problem" that should be handled appropriately by the best means.
Third, fish should not be fed other fish (here referring to the basic fish we are discussing, not specialist feeders). This is not some sort of animal welfare rant, it is based on nutrition. Fish are not healthy food for other fish in an aquarium. There is a vitamin issue (sorry, can't remember precisely what) so this is not something one should encourage. While it is true than in the wild an angelfish would easily eat a neon tetra or similar fish if they encountered one (which is highly unlikely to begin with, given their respective habitats), small fish are certainly not the staple of their diet. Insect larvae, worms and crustaceans form the principal food of almost all the fish we maintain in aquaria, and these ensure adequate nutritional needs are met.
The fact that angelfish will eat (usually quite easily and readily) small linear fish in the aquarium is simply because the fish are being confined in a small space (compared to what nature intended) and certain habits emerge or are significantly heightened that would be minimal or non-existent naturally.
Byron.