Angels are very peaceful, that is why they're great for community tanks.
Cichlids are alway seen as killers, and some of them are. Most of them though, only go after fish for two reasons.
1: They see them as food
2: They see them as a competitor.
Food would be something able to fit in its mouth. For an adult Angel that might be say, a neon. It's a question of time before the Angel reaches a point when it starts eyeing the neons and thinking maybe, just maybe. His stomach growls.
Meanwhile the neons are doing their thing, and the Angel keeps getting bigger. It gets to a point where the neons notice that gleam in the Angels eye and think maybe, just maybe. Their shoulders tense.
A Serpae tetra is larger, plus it has a laterally compressed (narrow and tall) body that makes it more difficult food. The Angel instinctively knows this and doesn't see the Serpaes as food. The is also not a competitor as say, another Angel or other cichlid might be. The Serpaes are ignored. The Serpaes, recognizing the lack of threat, happily go about their business, as relaxed as Serpae can get.
There are hundreds of characin we can use in place of 'Serpae tetra', all of which you don't need to rely on 'luck' to work. There are actually very few tetra's that are not suitable for Angels. Neons is one of those few. We can rely on luck though. They're only fish.