To be honest after the advent of farming most migrations had little effect on the genetic makeup of a region due to the existing sedentury population being significantly larger than the migrant groups.
In the case of Italy even after depopulation there would be millions of people living in the region while the invading germanic peoples would number a few hundred thousand at most and would be arriving over a long period of time.

Concerning the OP I would be very hesitant to link culture and genetics in such a way (the Irish and Basques are genetically very close but in terms of culture miles apart) and to attribute things such as armour styles to it is plain wrong.