Your LFS will almost never tell you when something is too big. The juvenile would be okay I would be afraid of it outgrowing an 80g.
They will need a 6ft tank when they are full grown, the larger or better. Maybe it's not the answer you want to hear but there are plenty of dwarf angels that are equally as beautiful that would fit just fine. My favorite is the Coral Beauty or Flame Angel. Do not confuse Flame with Flameback, Flamebacks are more difficult to deal with.