This may be by design, but I am playing as Germany, and confronting the USSR, and the Soviet airforce is clobbering me defensively.

And the territory of the USSR is covered with farms.... no villages or hamlets.... just farms...farms farms farms.

and btw, this mod is a lot of fun to play

lol the russian are good farmers :) and in 1941 they had biggest airforce in the world (it doesn't change the fact that half of airplanes (mostly obsolete) was destroyed in first days of barbarossa operaton on airports :))

seriously Russia needs to be more balanced - they are quite unplayable in the beginning - i think theire econommy sucks to much (even looking on historic aspects of Stalin's industrial and agricultural reforms - and farming it's because they don't have villages on the start and i suppose that ai thinks it's to late for building them) and it makes irritating (the same goes for playing french wich can't stop italian invasion :/) but slowly we'll getting with mod and scenarios from rough diamond to brilliant :)

I noticed that medium tanks can be researched before tank destroyers
Shouldn't it be a prerequisite?

It depends - it's very hard to make techtree good for all countries - for example soviet union started production of heavy tanks in early 30's, about tank destroyers - first one (for germany) appeared on Pz1 and Pz2 chassis = so quite soon