You definitely don't want to put the light in with the monarchs - they will fly up to it and get overheated in it - but putting the cage underneath shouldn't be a problem. To get your monarchs to mate, you will need to give them about 13 hours of "sunlight" every day. We imitate this with special full spectrum lights. Your growlight might work, but if they don't seem to be mating you might need to add more lights. We also recommend putting a humidifier in the same room; they do need a good amount of humidity. Lastly, you should make sure they have an area at least 2x2x2 feet square to fly around in; this seems to be the smallest size to get them to mate. Although it is possible to keep monarchs all winter, it does tend to be a bit of a challenge. We even run into occasional problems here!