When choosing a plant to pot up a great tip to keep in mind is to select a plant that highlights the strength of the pot and creates a balance.
Check how fast the plant grows. You don't want to put a fast growing plant in an undersized container. It will become root bound and needs to be re-potted within a year. You can continue to trim it but it will look like a small sock on a big food. Out of place!
Check out the root system. Is it trailing or short and stubby. Make sure there is room to handle those roots without over crowding. Herbs tend to travel so make sure that you keep them in smallish containers and keep them trimmed that way they won't take over the whole yard but stay contained in the planter.
Short on cash? How about using plants already around the yard. Bedding plants look good in containers. Know where you will put the planter once it's planted? Find plants that will like the amount of sunshine the pot will get. This is important because a shade loving plant will not like being potted and placed in an area where direct sunshine will hit it.
Try using ground cover from around the yard to pot. Forget-me-knots or campanula which tends to be invasive. Do you plants bloom in colors. Try limiting a container to one color for a unified look. If you are into a more eclectic look then pack all colors into one pot and have an explosion of color to view.
Harmonizing plants? Well, it's pretty much up to you. Do you really like one type of plants like succulents, cactus, flowering greens or all green and no flowers? The more complimentary the planters look the more natural the grouping will appear.