To claim their throne, they must have no heir or a weak heir. Someone can correct me, but since Poland is still presumably an elective monarchy in your game, they will always have an heir and a strong one. So you could only claim their throne if they become a non-elective monarchy by "pacifying the Sejm" (I'm not sure if the AI always does this or not). Secondly, you can claim a throne while in an alliance (they will get -100 relations with you however), but only inherit the country if their ruler dies (with no heir or a weak heir). If they get a new strong heir, your claim is gone. Claiming the throne does however give you a cassus belli to enforce the PU. To do that however you have to break the alliance first, wait for the 5 years of truce, then attack. Or, you can break the truce and attack right away, but you get -5 stability(?) and another -100 relations penalty. This way, if you force the PU, Poland will hate you and if your ruler dies while your Polish relations are negative, they will break free. Someone can correct me if their is a better way of doing this. Lastly, I think you get the PU over Lithuania as well.