If you have two mediums below the one they are filling with honey, you have ample brood space in the two lowers. If you added supers they will fill them only after the majority of the brood chambers are filled. For the most part the brood boxes always a mixture of brood, honey, and pollen. I also do not like to add more than one super at a time. As the excess space makes it hard for the bees to control small hive beetles. when a flow is on I add another when the top is 80% full.