I expect the cat on the Brazilian carrier is not powerful enough to launch a J-15.
You are right...it is not.
What they plan for their new carrier at this point, I am not sure, but IMHO, those decisions are already being made in any case.
It is fairly evident that the Brazilians will work with the French to design and then build in Brazil their new carrier. And they have already chosen the Grippen for their future aircraft and will almost certainly go for the Sea Grippen for their carrier aircraft.