Answer: I think your tropical garden plan has a lot of merit and it sounds as though you know enough to enrich the soil for good plant growth and provide some additional humidity through a mister. A waterfall will also help humidify the air.
Some plants to consider for use as understory include Alocasia, Calocasia, Philodendrons, Anthuriums, Crotons, Ti?s, Impatiens, Begonias, and a whole myriad of other colorful and delightful types of plants such as ginger, fuchsias, bromeliads, ferns, orchids and dwarf daylilies. Between the foliage shapes and the colorful flowers, you'll have an amazing tropical retreat.
Best wishes with your new garden!
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