Pretty much yes. The major difference being that when Brainiac Five created the antidote to Mon-El's lead poisoning and weakness, it had the side effect of allowing Mon-El to retain his full powers under a red sun.

Plus, while Superboy, Supergirl and Mon-El were all classed as full members, only the latter was a permanent resident in the 30th Century.


And the rule was waived under special circumstances on other occasions. For example, Lightning Lad was killed during an early Legion adventure. His sister Ayla joins the LSH as Lightning Lass. Very shortly after Lightning Lad's return to life Ayla loses her electrical powers, but gains the power to cancel the effects of gravity. She remains with the Legion as Light Lass for years, until an accident restores her original powers. The 'unique power' rule was waived to allow both her and her brother to remain on the team.