This is a very good question.
I am personally very fond of Crown of Thorns (Euphorbia milii) as underplantings; if you get the small kind they will stay short. Other good plants to consider are Bulbine, any type of Aloe or a small Bromeliad such as Neoregelia.
I don't have my Plumeria in pots, but all of these are under my Plumeria trees. You want something that doesn't get too tall and will like the same or similar conditions.
