Several things to consider.
Plant size - height and width
Growing requirements - water, pH, fertilization.
DITTO Pinksand. No straight lines. Stagger your plants.
Bloom time: most perennials are summer bloomers. Some early, some mid summer, some late. Arrange your plants so there is constant color all summer.
Is this bed intended for 360 degree viewing or front viewing only? If 360 then the Knockouts need to be in the center of the bed with you other plants growing tallest to shortest from the center out. If front view then graduate the height from back to front.
You may want to include some prostate or clumping plants. New Gold Lantana is an old, tried and true sprawling perennial. Lots of bright yellow flowers. Maybe include Prostrate Rosemary. A nice touch of green and ready to use in cooking. Ornament Sweet Potatoes are a great pop of color and a very nice ground cover for between your plants. The purple and chartreuse are my favorites. A beautiful color combination. Not a perennial but a few planted in the spring will cover a large area. In south Louisiana they come back every spring unless we have a stupid, crazy cold snap. The vines will root with contact to the soil and actually make a potato. The potatoes over winter and produce new plants the following year.
Any way - have fun and good luck.
BTW - love Knockouts both for their easy care and very long bloom. Here they bloom year round. Do keep in mind that they can get very large. Consider mature size when you plant.
Post pics when you get it done.
Elizabeth - or Your Majesty
Living and growing in Lafayette, La.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant. ~Author Unknown