Answer: Based on your description I think you may already have enough plants in that area. The reason is that the ones you have already planted will grow taller and wider in in time and fill in. It would be a shame to remove one later on as it reaches its prime just because it is too crowded.
If you really want to add to your planting, I would suggest you take a sketch and/or some photos (along with measurements) of the overall area and consult with your local professional nursery staff and/or your local county extension. They should be able to advise you based on a more detailed understanding of the planting area (size, plants already planted, wind exposure, sun exposure, soil type and moisture levels) and your overall design goals.
In the meantime, if you need something to visually fill in the open space while you are waiting for the trees to grow, you might enjoy some of the newer low maintenance landscape roses such as the KnockOut series. These bloom over an extended period and require very little care. They should do well on the west side of the house.
I hope this helps.
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