Answer: The rule of thumb with a weed and feed product would be to wait until the new grass has been mowed four times before applying it. Carefully read and follow all of the label directions.
Ideally, you would run some basic soil tests and develop a lawn care program based on the results of the tests, along with knowing the type of lawn grass you are growing. Your local county extension should be able to help you with the tests and interpreting the results and developing an annual schedule for lawn care. They should also be able to tell you which kind(s) of fertilizer you need to use and when to apply it to get the best results. (Often, we tend to overfertilize and often we do not lime correctly.)
Some naturally small growing shrubs you might consider include the summer blooming spireas such as "Limemound", small varieties of barberry such as Crimson Pygmy, and dwarf nandinas and the smaller rhododendrons such as PJM as well as dwarf boxwoods. You can also use yews and hollies but these may require constant pruning to keep them that small. Your local professional nursery staff and also your county extension may have other suggestions based on a more detailed understanding of where you want to plant the shrub (sun or shade, wet or dry soil, windy or sheltered spot) and your design goals.
I hope this helps.
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