Chris, you will want to keep those plumeria seedlings going with bright light, watering when the soil dries out a little, diluted fertilizer (1/4-1/2) monthly, and good air movement. Keep the temperature above 55F at night. It doesn't matter how hot the greenhouse gets. Plumeria love the heat. You might see the lower, older leaves yellow and fall off during the fall/winter, but that should be about it. Next year you can allow them to go dormant.
