Looking at the look of Winged Hussar it could be restricted only as bonus for civilization, not as team bonus. But It’s AoE II, everything can happend.
You also mentioned bigger mill. It might be like you said to drop off food, wood, gold and stone. It could also train animals like pig or cow. It’s at least 3x3 big. Maybe It could garrison vills like Khmer houses.
There will be 3 versions of “winged” Hussars now. So I see no reason for these not to be part of the civ for time frame reasons. Likely as an upgrade like it’s the case with Imperial Camels.
If we are talking about times before Definitive Edition, Lithuanians represented as Slavs was right move. I mean, the Lithuanian nobility were called Boyars and Ortodoxy as unique tech was spots on too.