I think the reason it costs population is because now germany has an ‘infinite native’ shipping from age IV (14 habsburg allies). This shipment does not cost population. The ‘Army of the Circle’ card also sends royal house allies at no population cost.
I think it would be nice if on top of that it gave a couple of associated buffs in the embassy, and/or gave you a bunch of respective units when choosing the ally.
It would be great if it didn’t cost population, but it would be broken in the late game.
Maybe I could make them cost no population until age IV. In case you don’t have this card (infinite native) in your deck, allow these allies to cost no population.