I often find it useful to useful when selecting plants for a garden composition to look first at their forms and relative sizes. It's possible to give more unity to a garden, even when using a large variety of different plants, if they have similar forms. In this photo, all the plants have arching foliage, which ties them together thematically. On the other hand, they also have well defined contrasts in foliage color and texture. The most dominant plant is the largest, the fine textured tree fern, Dicksonia antarctica which is the focal point of the arrangement. The blooming bromeliad which surrounds the tree fern gets to play seasonal notes when it comes into bloom. The foreground herbaceous perennial with the silvery foliage is also planted in mass and provides the frame for the others, yet also gives year round interest by brightening up the shaded entry garden year round. Color is part of the total composition, but clearly is playing a secondary role to the leaf textures and different scales/sizes of the plants. These plants were selected because they will never outgrow the height relationship set up here, an important design consideration if you are using a broad mix of species and want them to play nicely together longterm. This aspect of time and how it supports the design is something that only comes with direct experience with growing the plants, and can't be faked or rushed; it comes with time and careful observation...

Here is a link that might be useful: The Three Archers, complementary forms