If all original houses would ally and fight back Gallia, I think they would do it.
The combined power of Rheinland, Liberty, Kusari and Bretonia? I think Gallia could not stop them.
Yes, gallic military is very strong, has powerful weapons etc. But the Rheinland ships are very heavy, too. Liberty ships may not be so strong, but they have way more.
Just look at all the ressources of Bretonia and Rheinland, or the technology of Liberty and Kusari ...
But the problem is ... they do not ally. As far as I know, Gallia has no problems with Kusari and Rheinland, because both didnt trait them. Only with Bretonia and Liberty ... so why should Kusari ally with their enemy Bretonia against their potential friend Gallia? Same for Rheinland against Liberty.
And you have some nationalistic things to mention.
How many Bretonians would be happy to see Kusari warships defending New London? Or Rheinlanders shipping cargo to build some Liberty Dreadnoughts?