I really don't want to turn all of this to a pointless bickering, but you are (by mistake, due to your excessive desire to prove you're right, or on purpose) incorrectly quoting your own sources.
For starters you are saying that Brandenburg reached 2 mil people in 1940, but you are forgetting to mention that data is related to the State of Brandenburg (corresponding to the in game's province Brandenburg) and not Prussia, which had 25 mil people in 1871 (little less than France, little more than Austria-Hungary).
12,000 men isn't "armies".
12.000 men were German casualties, as is cited in the article you quoted. After initial heavy loses by the Austrians (nearly 300.000 dead in the first couple months of invasion on Serbia), Germans took over the command and formed Army Group Mackensen (Heeresgruppe Mackensen, which included the German 11th Army, Austro-Hungarian 3rd Army, and Bulgarian 1st Army). Only the German 11th Army had 120.000 soldiers...
That Army Group was lead by Field Marshal von Mackensen, who, after the occupation of Belgrade erected a monument to the Serbian soldiers who died defending Belgrade, saying, -HIER RUHEN SERBISCHE HELDEN- - "Here rest Serbian heroes".
Austria and Germany? How the hell does being conquered and assisting a successful partisan movement count as "beating up"?
Ottomans and Austria. Serbia defeated Ottomans decisively in a series of wars and won independence and territorial expansion, without any help from European superpowers. It also humiliated Austria by repelling their attacks and winning first major battles for the Entante in WW1. Both Austria and Ottoman Empire ceased to exist after the WW1.
Whose weapons? Yugoslavia came into existence after the Treaty of Versailles, and was supposed to be a union, not an expansion of Serbia.
Yes, but it was won with blood and it came to the existence only because Serbs viewed their sacrifice as a mean for a higher and just goal - liberation of all South Slavic peoples while Allies actually offered something else).
It doesn't say anything about the Partisan composition being propaganda. And propaganda for whom? Those figures came from Tito himself. Also, the 77% statistic you provided came from a brief period in 1941; by the end of the war, the Partisans in Croatia were over 60% Croat.
Propaganda from Tito himself. Communists forced the mantra of Brotherhood and Unity among South Slavic peoples so everything had its "national key".
National teams had proportional number of players from each republic, state administration positions were proportionally divided, medals were awarded by "national key", etc. So the liberation movement had to be equally divided as well.
So, from Tito's little speech to Churchill in Caserta in 1944:
We need to draw the fact that here in the ranks of the People's Liberation Army and Partisan Detachments of Yugoslavia, from the very beginning until now Serbs are in the vast majority. Serbian, Montenegrin, Bosnian and Lika partisan brigades, which are composed almost entirely of Serbs, today lead a ruthless struggle not only against the occupiers, but also against the Chetniks of Draza Mihailovic and other enemies of the people.
... passed very little time for that to be changed to: Partisans from Serbia 44%, partisans from Croatia 30%, from Slovenia 10%,... Neglecting the fact that the people from were actually Serbs born in Croatia, Bosnia, Slovenia,...
And regarding the Serbian armed forces in the timeframe of the game - if you wanted to actually read something about that, I'm sure you'd manage to find everything that you are interested in. Here:
http://en.wikipedia.org/w/index.php?title=Medieval_Serbian_army&redirect=no
http://en.wikipedia.org/wiki/Hussar
http://en.wikipedia.org/wiki/Polish_hussars
http://en.wikipedia.org/wiki/Military_Frontier
http://en.wikipedia.org/wiki/Battle_of_Kosovo
http://en.wikipedia.org/wiki/Serbian_Despotate
http://en.wikipedia.org/wiki/Order_of_the_Dragon
http://en.wikipedia.org/wiki/Jovan_Nenad
http://en.wikipedia.org/wiki/Starina_Novak
http://en.wikipedia.org/wiki/Freikorps
http://en.wikipedia.org/wiki/Ko%C4%8Da%27s_frontier_rebellion
