Again, Father, your growing conditions are so different from mine, I perhaps can't give you good answers to your questions. Guardman comes in lots of sizes, and since you want to grow plumeria, you would want the largest possible size that you have room for and is within your budget. Plumeria are trees and will get large. I see no reason for you to take up valuable space putting shelving in that mini-greenhouse. Two of the advantages of having an indoor greenhouse is that it will raise the humidity and allow you to have more heat where you are growing the plumeria. AC works against you in that regard. You might have enough heat simply from using your lighting but you might want to put a low-wattage heat-source inside. You'll need a thermometer to hang inside the greenhouse to keep up with the heat-factor.
I have never grown using these mini-greenhouses. I don't know whether plumeria will grow well with constant temperatures. That's a challenge I see. My greenhouses heat up during the day and cool down at night. I don't quite know how you can mimic that with an inside greenhouse that gets no sun.