Answer: As far as looks, it is totally a matter of personal taste what looks good together. In the photo, it does look like a juniper to the right and possibly a lanceleaf coreopsis in the rear for the yellow. You could use black eyed Susans (a bit more orange perhaps) or daylilies or even yarrow for a similar backdrop.
Junipers and roses (and black eyed Susans) grow well in full sun so they could be mixed or planted next to each other.
You will need to space them carefully so the junipers have enough room to spread out over time (they grow slowly but they do grow) and so the roses also have enough space. Roses need good air circulation so you do not want them to be too crowded. I would suggest you plant them in drifts, meaning an informally shaped group(s) of junipers and a separate group(s)of roses.
The reason for suggesting this is that the roses will need more watering than the junipers and keeping them in a group will facilitate that chore. On a slope, the soil is usually very well drained. This is perfect for junipers but may be too dry for good blooming on the roses. Roses perform best and bloom better when the soil is evenly moist yet well drained, meaning damp like a wrung out sponge.
So, you might want to plant the junipers at the top of the slope and the roses at the base, simply because the top tends to be naturally drier and the lower part of the slope is usually naturally moister.
And, when they are new, both will need to be watered for the first growing season or two until they become well rooted and established.
I would also suggest you consult with your local professionally trained nurseryman. They may have different thoughts or suggestions based on a more detailed understanding of the growing conditions on the slope and your overall design goals. Have fun with your planting!
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