Well..there's not really too much to do, right? it's not like the two Gundam Universes where we have to change a bunch of stuff around, it's just a generic Planet Earth. I absorbed Mazinkaiser SKL into Gurren Lagann, so Machine Island is just a large section of the Dark Continent where savage humans have managed to hold out against Lord Genome. I know that an empire invades or something in Getter Robo, and so does one in Heroman.

So why don't we base the jumping off point for Super Robot Earth on that? Gurren Lagann and Mazinkaiser SKL's plots start on The Dark Continent together, meanwhile the events of Getter Robo and Heroman start when the Skrugg (eventually) invade, as does the empire in Getter Robo/stuff that happens, which alongside the Dimensional Beasts is what brings Crowe to fight alongside them. 

So the initial crews will probably start out as :Joey's Gang/Getter Robo Crew/Crowe, Ken/Ryo/Tsubasa/Kamina/Simon/Yoko, and the SEED crew/0079 Crew. Eventually they all come together, though.