Answer: The answers to your questions depend in part on your "taste" in landscaping.
These shrubs can look a bit untidy or gangly, so planting them in a clustered group on four foot centers helps to hide the straggliness and make them look fuller earlier in the season. In cold winters these shrubs will die back to the ground and need to be pruned off short. In mild winters there will be a certain amount of winterkill in the branches which will need to be trimmed off in late spring. And since they start into growth in late spring, the hedge effect may not happen until mid to late summer.
For seed starting, in my experience it is generally better to start them in a pan or in little pots where you can monitor their progress and then plant them out when they have reached some size. This way too you can select the most robust healthiest looking seedlings to pot on -- and to share with friends if you have too many. Incidentally, the packet should indicate the expected germination rate for your batch of seeds under optimum conditions.
For a groundcover, you have many choices. These shrubs don't make terrribly dense shade, so plants for sun to part sun would do well. Some gardeners prefer to maintain that silvery look and use silvery perennials such as artemisia, dianthus, or nepeta. Others prefer to add hot colored contrasting plants such as zinnias and marigolds or bold daylilies. Still others look for plants to bloom earlier in the season, before the shrubs look like much, such as spring bulbs. Another approach is to add more popular butterfly plants such as asclepias, purple coneflowers and Blackeyed Susans. If you plant the perennials thickly you will have a "groundcover".
For a traditional evergreen groundcover effect, spreading forms of thyme are nice as are creeping phlox or even vinca minor. Alternatively you could simply topdress annually with compost and mulch to suppress weeds.
Enjoy those butterfly bushes!
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