Well first of all Italy has a number of traits, cities common with the Roman Empire. Second, HRE was not just Germany... HRE Encompassed parts of Spain, France, Hungary Austria. Italy on the other hand is nowhere near the size of the Roman Empire. In fact Italy splintered, and has yet to gain at least the military prominence of Roman empire.. Italy's performance in both WWI and WWII (luckily for the rest of us) was lackluster at best... Do you propose italian military units be at -100% to greek phalanx and -100% to what ethiopians wield, since that would reflect reality would it not, and while you are at it at -100% against french units led by Napoleon, and at -75% to British Navy, and at -100% to HRE units.
Concept of United Italy, after the fall of the Roman empire did not come around till late 1800's, until then Italy was a bunch of city states. After all would it not make sense to make Venecian, Milanian, and Florentian civilizations more than an Italian one? They certainly made more political and military impact over a greater period of time, too... From the Mongol Invasions, through rennaissance.
German were always a part of either HRE, or independent, and were eventually temporarily united under Frederick the Great, and their troops played prominent roles, in American Revolution, Napoleonic wars, etc.
I mean Hungarian Empire, would make more sense, as they did manage to Stop Mongols, on the second run through, first time around they were wiped.
Russians could use another leader, Alexander Nevsky, it was he who led Russian charge to stop teutonic and swedish crusades to force Russians to continue fighting Mongols, or at least to assume that fight.
That's the real reason crusades ended... Europe was trying to stave off Mongol invasion. Had Ogadai not died, and had bubonic plague not occured, ravaging it's way from China (part of which was under mongols) via trade routs to Italy (city states of which sold slaves captured by Mongols to Egypt, thus making their wealth and setting the scene for Rennaissance), and then working its way to England via Italian merchants... We'd all be speaking Mongolian now...
Anyhow, this is all besides the point.