Only the second Proty was ever a member of that august “Legion”.
Chaim Mattis Keller
Legion-Reference-File Lad
Unless the most recent reboot changed something, Mon-El/M’onel/Valor would have to have a part in at least the origin-of-the-team section of any Legion movie, as since Crisis, he’s taken the place of Superboy (who no longer exists - at least in the form used in LSH) in the inspiration/creation of the Legion - and, IIRC, the galactic government that makes it possible, but it’s been a while since I’ve read it, so I might be misremembering that part.
Chaim is correct about only Proty II being an actual member of the LSP, though Proty II was just a pale copy of the wondrous nobility and honour that flowed through the lymphatic pseudopods of the brave Proty I.