Right at the beginning, the best bet for Poland diplomatically is to form a coalition of small powers. Generally speaking, Hanover, Bavaria, Moldovia and Saxony are willing to join a Polish based alliance. Only Moldovia is useful on wars on the Eastern front since the others lack a common border, but if Brandenburg or Austria want to start something, the small allies will take much of the heat for you.
In terms of religious revolts, the best policy is to change your religion settings so that tolerance for Muslims is set at zero. This will me Keremetjung will revolt frequently, but you won't have to worry about the rest of your provinces. If by chance you get settlers through random events, the constant revolts will keep the population of that province low enough to forcibly convert.
One last thing about playing Poland - converting to Protestantism is a very, very good idea. It gets you a steady supply of colonists to convert Muslim territories, increases the amount of money you generate and will boost your diplomacy with the German states, allowing you to diploannex them eventually.