I am at Southern California. I need ground cover for 2000 sq.ft. with 60º slope and help for choosing the right flowering low ground cover. There are 30 rose plants at present, so prefer ground cover which does not compete with roses.
With your zone of 8-9 this page offers some excellent choices: http://homeguides.sfgate.com/evergreen-ground-covers-zone-8-34125.html.
the "Blueberry Muffin" carpet bugle (Ajuga 'Blueberry Muffin') is a fast-growing ground cover that produces blue blooms protruding above the evergreen foliage on 8-inch spikes.
The common name of iceplant (as mentioned in the comment) are not all bad. For example, the Delosperma cooperi (Hook.f.) L.Bolus (http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=b464) is an excellent flowering choice.
You need a ground cover that is higher to help shade the soil to minimize weeds. Lonicera pileata or Privet Honeysuckle has strong horizontal branching, green, glossy privet like leaves and is 18-24" up to 3' in height equal or more in width. To show off your roses you should only plant one type of plant so as not to detract from your roses and tie everything together in a pleasing composition.
I've used this plant in similar situations and was thrilled by its effect. Plant in staggered rows or triangles keeping the roses with 3-5' radius distance. Mulch after planting with 2-3" decomposed organic mulch, NOT BARK and NO PLASTIC. At 60% slope it will be necessary to terrace and berm each plant to catch and hold any irrigation/rain until established. Keep mulch away from the graft and wood stems of your roses.
You've mentioned 30 rose plants, but not said what type of roses, (HT, floribunda, shrub, ramblers) so its quite difficult to suggest ground covers that won't interfere with them. You'll have to work that out for yourself, given the lack of info on that issue. The fact the plants are on a slope probably means it's quite a dry area, particularly at the top, so careful selection should be made between plants on the following list which are drought tolerant and those which prefer shadier, damper locations:
Arctostaphylos 'Pacific Mist'
Arctostaphylos uva-ursi (needs shade and a bit more moisture)
Arctostaphylos uva-ursi Radiant Manzanita (needs shade, as above)
Ceanothus gloriousus 'Heart's Desire'
Stachys byzantina Silver Carpet
Mahonia aquifolium 'Compacta'
Zauchsnerias - 'californica' and septentrionalis varieties
Salvia somonensis varieties
Eriogonum umbellatum and ursinum
Salvia 'Bees Bliss' and S. gracias
These are native to Southern California, but some do better right on the coast, others inland, and you haven't said if you're close to the coast or not - the list mostly excludes those that prefer beach conditions. If the area is going to be irrigated, other plants can be included - note that Ajuga (mentioned elsewhere) prefers shady or dampish conditions.