In a 55, if the tetras are in a school (at least 5), and you start with small angels, it shouldn't be a problem. The angels will grow up with the neons, and won't see them as food. Or at least not for a few years until the angels are truly good size-at which point the neons will be getting on towards old age anyways. I've added little cory catfish with medium angels which were large enough to eat them, and there wasn't a problem.