They belong with Tanks and fighter jets?
I arrange my units the same way they are in the game.
Modern is post-WW2.
http://forums.civfanatics.com/downloads.php?do=cat&id=23
Industrial = 1700 AD - 1900 AD
Modern = 1900 AD +
From this, you could put your Polish cavalry in Modern instead of Industrial.
You decided otherwise. Fine for you. But perhaps others don't want to follow your standard.