I think a large part of the diplomacy they have is down to their shared hatred of the corsairs with various people...
I can't imagine that a corsair backed group (such as the Gaians) would want to have anything to do with such a renowned bunch of corsair-haters as the mollys.
I would regard the common enemy thing a major reason for the alliance with the Hessians as well. Although that said, there isn't much the 'casts and hessians can do from their respective corners of sirius to help the mollys, other than providing ships (as the 'casts do) and another enemy for the 'sairs (as the Hessians do).
That said, the diplomacy for a lot of factions isn't perfect, and I'm waiting to see what it's gunna be like come 4.85.