To be honest I have no desire to form Russia with Poland. As the game stands now the area where Russia should be is a huge mess. The Hordes have neutered Muscowy and the Teutonic Order has done the same thing to Novgorod. Both of those countries have only one province each now and the rest of the area is filled with other one province minors that the Horde, Sweden and TO curb stomp every couple of years. Even if they conquer them another rebellion pops up and they split off in short order.
As for me as Poland I've managed to expand a LOT by both diplomacy and warfare. Moldavia was diplo annexed after they conquered Transylvania. Mazovia also agreed to join me peacefully. I took Warmia from the Teutons and Carpethia from the Hungarians. Then I managed to colonize and keep the Crimea and two other Horde provinces that border it and whose names I cant remember....or spell in all liklihood. ><
Best of all, in a series of small wars I've been called into by various allies (Sweden, Burgundy, Silesia, etc) I've managed to maintain the PU with Lithuania and build a sizable army & army tradition in process.
It's only about 1465 now and my inflation is brutal but Poland is a power to be reckoned with...just not in it's traditional areas. Am wanting to expand westward towards richer lands but a Bohemia/Austria/Brandenburg alliance seems to have been created solely to prevent that. Will just have to see how that goes I guess.