Those Polish Nobles at the start are very powerful. They're pretty cheap to support too so they'll have tons of them when you fight the Poles. You have to basically survive using Khazaks, Boyar Sons and creap spear militia with a crappy economy at the start of the game. I think conquering the coastal provinces and only a few of the inland provinces is a good start. Just don't get stretched too much.

Russia gets really easy in the mid to late game. Dvor and dismounted Dvor are just extremely awesome. If you control Stockholm and a few coastal provinces there, they give a ton of income.