Sadly, only Earth-Four was discovered during this period, with its Charlton heroes safely in the fold ready to be assimilated in DC proper. The Mighty MLJ gang took a few decades before they came around to play in the same sandbox with the big guns.
Couldn't they have just all stayed separate, giving us different takes on how a limited number of heroes championed good on their respective worlds? Rather than big gang scenes? Alas, assimilation is more than just a science fiction concept.