There are MANY choices for grasses in zone 8--are you looking for perennial or annual grasses? And how big? What other plants are in the garden that they will be complementing, and how much room do you have?

There are several perennial Pennisetum species that go well with tropical landscaping, but one of the best is an annual (even in zone 8), Pennisetum setaceum 'Rubrum' with dark purplish-red leaves.

Miscanthus species are also generally hardy and go well with tropicals, but most tend to be on the large side.

And although you may not think of them as grasses, many hardy bamboos will very nicely complement a tropical-style garden, as will the giant grass Arundo donax.