Spacing for Pacific shore juniper (Juniperus conferta 'Pacifica') depends on how much money you have, how fast you need total coverage, and how many holes you wanna dig.
Commercial specifications are 3' on center, staggered rows horizontal to the slope. This allows the plant to fill in economically without having years of weed control while it grows thick enough to block many (never all) competing weeds.
Full sun is essential to keep any juniperus species happy. Soil richness is not necessary, however, waterlogged soils will kill them fast. When digging holes, only go as deep as the plant in the pot, and at least twice as wide all sides, breaking up the soil so the roots can spread.
Mulch area well with aged wood chips 2-3 inches deep (aged 6 months or more) or 4 - 5 inches pine straw. If a large hill, spray weed control is used, but I prefer hand weeding on small areas.
If erosion is really a problem while things are getting established, look up erosion control things - temporary ridges of material horizontal to the slope.
Only use a light application of fertilizer after growth is established.