do you know how to post pictures? would help alot.

palms stay pretty small for about 5 years so they would be the understory until then. i always like cannas, colocasias, alocasias and musas around pools and water

Some palms stay small forever like lady palms. They like shade, tolerate frosts, but downsides to this plant is they grow very slow and are expensive even at medium sizes.
Bananas would look fantastic! There are dawrf bananas that are edible, but they need sun to thrive. Philodendrons would also be a fantastic addition and are more shade tolerant. Actually many philodendrons prefer shade. Peace lilies make great understory plants but must be in full shade. Mine are under a tree (during the summer months) and get no direct sun and they love it. When they saw the sun for one day (wasnt even that warm) they were pretty burned.
Cannas are also great and multiply fast so if you want a lot of space to be filled up quickly, cannaswill do that for you very well. There are lots of really nice colored elephant ears and alocosias. Some stay very small and others have leaves considered to be some of the largest on the planet.

Heliconias are a beautiful option as well as bromeliads. Pineapple plants arent attractive when not in fruit, but when they are, they give a very tropical look. Even in the tropics, I would plant it in a nice pot as an accent plant because I just dont think they look that great in the ground unless in flower.

Good luck!