Well since you already have the fish you mentioned, your tank will be pretty close to fully stocked with a pair of angels in it. Neon tetras are angel's natural food in the wild, so even with a school of them you're probably going to have the angels eating a lot of them. You'd need at least a dozen neons to get them to school well enough for it to be an effective defense mechanism against the angels, and effective is relative since in the wild angels eat neons even though they're in huge rivers and have lots of room to escape and school in the hundreds. Long story short it will work until the angel's mouths are big enough to fit around a neon.
