In the 'T'Book' there is detailed a 'Type M' (subsidized liner). Using a 600dT hull, it has J3 & 1G.
In addition there is the 'Type C' (mercenary cruiser). Using an 800dT hull, it has J3 & 3G.
Also there is the 'Type T' (patrol cruiser). Using a 400dT hull, it has J3 & 4G, which would seem to contradict the "400-ton to J2" limit you mention above?