Well, mercenaries can, especially over the long term, be more costly than they are worth. But you should not refuse to hire mercenaries point blank if you can make gains with them and strengthen your position.

One thing worth trying for the Russians will be Woodsmen. I believe the Livonia-Lithuania-Poland area can produce Woodsmen, with a valour bonus in Poland IIRC. These are quite cheap, charging, line-breaking infantry. Can you not produce them in Livonia? Maybe a dash for Sweden and Norway would be worth a try.

Also, I would expect that you would be able to build armoured spearmen with the appropriate developements.