I dont know the exact numbers of the Kusari fleet (example: bretonia - 30/500/90k) but if their ships are more than the bretonians, in RP it makes sense if Kusari declare war on Gallia in the distant future. Kusari can see the truth how Gallia tolerate the weak (right now house bretonia) and foresee their future as next target which could make them slaves. I doubt the kusari people would let that happen...
Gallia will have to fight on two fronts with two Houses instead of one, which will bring more activity to both Gallic and Sirian factions. The main battlefield wont be just "Edinburgh-Leeds" but Taus and Kusari (maybe Kyushu).
Such change could give the kusari lawfuls and unlwafuls factions something to do. In the same time GRN faction and indies will have the chance to choose where to fight.