Planting annuals and perennials among established shrubs can be tricky. If you're constantly setting in and removing plants, the shrubs' root systems will be disturbed. Tough, carefree shrubs may not be bothered by such intrusion, but less rugged individuals may be weakened. In some cases, it's the "intruders" that suffer ― the shrubs take most of the available nutrients and water for themselves.
If you want a mixed bed of flowers and shrubs, consider choosing perennial flowers; they'll thrive for several years before needing digging and dividing. It's also a good idea to set out fairly small plants ― those in 4-inch or 1-gallon pots. These require smaller planting holes and so disturb shrubs' roots less.
If you're adding a ground cover near or beneath shrubs, make sure that both plants have similar water needs. For example, a thirsty lawn isn't the best companion for shrubs that prefer dry conditions.