This...lasagna retains it's heat once cooked and covered with foil. for a while. Plus it has to rest anyway before you cut into it. If you have a second shelf in the oven, put the kielbasa in there. Can't the pierogi stay warm in a covered pot while you fry the fish? Fish won't take long to fry. If you do decide to bread ahead of time put them on a wire rack so air can circulate and they don't get soggy although I am not sure I would do it ahead unless you are really swamped.
any extra pierogi can be sent my way.