Adding tylose to fondant makes more of a modeling paste than gumpaste. Depending on the fondant recipe (or brand) it could have some of the same ingredients as gumpaste but not the same proportions. If you have tylose, can you make tylose gumpaste? It's egg white, powdered sugar and tylose and will hold up much better in humidity than modified fondant.
If the humidity is really bad, I'm not sure if you'll need a de-humidifier or silica packs to help out. What are you trying to make?