Multiple threads on this, now.
But yeah, as GM of a CE guild for 8 years now, Mythic should scale in size up to 30, and if they want to go down to 15 or even 10 (if possible) that would be good, too.
Fights WILL be harder at the lower end of the pool, because you’ll have fewer people to do jobs. But it’s better than not letting people play at all.
And flex isn’t about letting new people raid who can’t get 20.
Flex Mythic is mostly about guilds that are ALREADY doing Mythic, which requires way more than 20 players, and have to regularly tell people every night “you don’t get to play”.
Telling somebody “you don’t get to play” is the worst thing in WoW.