So, from what little i understand, the Corps represent actual corps which fought in the war, the detachments are brigade level or divisional level troops.
Garrisons are regiments.
Ottos need to protect a very long coastline, the Austrians need to Garrison the coast/Italy likewise and Belgrade and if Germany pushed into Russia, then they have to garrison Minsk etc and in case their navy loses a lot of ships, Hamburg etc.
So, why not allow say 5 garrisons each to all major countries, Russia anyway starts with around 5 garrisons, so maybe Russia can get 5 more.
This won't break the game. As garrisons are useless in attack, quite weak in defense and cannot entrench, but they can hold the line for 1 turn till troops are operated. This part is Historical.