I think is his point was that, in a film, 6 people can be the Avengers, but nowhere can you call 7 people a "legion." Also, part of the appeal of the LOSH is that there are dozens of them, that's part of the coolness for them. So without that, they're less cool. The Avengers have dozens of members just because the team is popular.
I think if you build the universe right, with just a bit of camp and make the big moment to start the third act this huge recruitment where the work of the few founders has paid off and now there's 30 super powered teenagers ready to wreck shop I think you could have something really cool. To further reference GotG, it'd be a bit like Footloose. There's 3-4 main characters but in the end all the background teenaged characters join in the heroic action.
I'd focus on Cosmic Boy, Saturn Girl and Lightning Lad as the three leads. I'd actually center on Cosmic Boy and let Lightning Lad be the dynamic foil while Cosmic Boy is the more subdued introspective main character.
Brainiac 5, Triplicate Girl, Bouncing Boy and Invisible Kid would be the primary recruits, and they'd all have really bad downsides and appear wack (except B5, he'd just have a bad reputation), but then in the end show themselves to be utter badasses who helped finish the recruiting and bring in the biggest baddest cameo shot which ends in a final sequence much like the end of IM3, where a lot of LOSH cameo while the action focuses on the two or three leads.
My LOSH Roster:
Leads: Cosmic Boy, Saturn Girl, Lightning Lad
Co Stars: Brainiac 5, Bouncing Boy, Invisible Kid, Triplicate Girl
Minor Speaking Roles to set up next time: Karate Kid, Chameleon Kid, Laurel Gand, Colossal Boy, Timberwolf, Kid Quantum I
Then just cameo about 20 others, mostly the silver age guys but use the new Star Boy, XS, Gates, Sensor, and set up future stuff with Nemesis Kid.
But honestly... I don't know enough about the LOSH to have any useful commentary on their villains, which is weird cuz I've at least seen a few episodes of the cartoon.