If I want to use a fruit or mousse type filling in a layered, stacked cake, how can I be sure this will not seep into my cake and make it soggy? Do I just do a buttercream crumb coat with a dam? How long before I am ready to serve the cake do I need to remove it from the fridge?

Just make sure your dam is nice and stiff and it will hold in the filling....also make sure not to fill it with too much. Another tip, when using fruit make sure to spread a thin crumbcoat the bottom layer or the filling can seep into the cake and it could end up mushy.