I had a short experiment with this myself, with an airline based in Kiev, in another game. This type of 'experiment' is one of the things that attracts me to this game.
The obvious problem is that there is no embargo (in communist times), or heavy taxes (in early capitalist days) on importing foreign aircraft. Certainly at the current time of this game it would not be realistic for a USSR based operator to be using western types. Given the vast and unique nature of the USSR could this not be used in the game model? Some form of restriction in this area.
Indeed I think there needs to be a way of programming in a benefit for using home built types for all national airlines. French airlines used the Dassault Mercure; BOAC / BA got the Comet, VC-10, Trident etc. In reality they were not used in competition with 'better' types in the same way as can be done here.