The northern invaders could not have been so many as stated. My reasons: With no roads and no supply systems an army that big would have starved quickly. Romans attacked them in Noricum but would have been more careful if they were so many. The invaders could have attacked twice roman lands but choose not to. They achieved nothing against Spanish tribes or in Gaul.
About the reform: the romans had permanent forces probably for ever but for sure since they set foot in Spain, roman levy had defeated Carthaginian and Hellenistic mercenary armies so they had no reason to believe that going that way was any good, the recruitment pool could have been raised by increasing the number of roman or italic citizens, standardization (if it was a goal but I doubt it) could have been achieved by the way Athenians did, that is by using state workshops to provide weapons to the citizens, but still keeping the class structure, the number of potential soldiers was huge as proved by the next century of civil wars, romans had money to recruit a larger number of foreign auxiliaries in case of a temporary shortage of man, I know no reason for a roman shortage of man as the losses from barbarians were definitely smaller then those inflicted by Hannibal.
Both the reform and Marius consulships had more to do with political agitation in Rome then with anything else.