1. If you can't do it yourself, you have to hire someone. You can either hire a proper gardener that can do both demo and install, or just get a daylaborer or demo and do the install yourself.

2.plant selection is design. Almost everything in SoCal is evergreen. Since it is near a pool, I would avoid deciduous plants.

3. Design wise, I would go with a modern matrix planting for the entire thing. Layers and masses are old fashioned. Basically, I would choose at least 5 plants that max out around 3'x3'. Then pick an accent plant that is a little taller, it could just be a tall bloom. You randomly plant the matrix plants over the entire area. Then you put the accent plants in that field in an aesthetically pleasing way. Yeah, I know that last bit doesn't help much but that's where the art comes in.

A good resource is san marcos growers. They are wholesale so you won't be able to order from them, but you can use their website for a lot of good info. You can call up plants by type and then just go throuh the list looking at sizes. Pick out some shrubs that are the right size and right water requirements. Or grasses and grasslike. Salvias are definitely doable. Look for cultivars that are smaller than the species.

I took out 2 agapanthus quite a few years ago on a dry slope. They are quite the daunting task. Plants are easier to dig up when dead a couple months. Perhaps spray them with round up now (70 degrees for it to work) or last fall if you had known.
Salvias will attract hummers and bees. Honeybees won't normally sting but do you really want them around children and especially with a pool.
There are lots of natives that are not succulents. Pick up a Western Garden book or Southern Ca gardening by Pat Welsh. Both are great resources.
Since you are a newbie I would stick with just 3-5 types of plants so you have less to learn and worry about. You have a busy enough life. You might look into a couple Ceanothus like Concha, but I'm not sure how they do around pools and the chlorine if splashed.
You might think about painting the wall a pale color to soften it rather than the bright white.

You are going to have to come up with a strategy to keep your Great Dog from destroying the plants first. Some selectively placed hard design objects (fountain, boulders) can help to reroute his path.

For arrangement, a simple base group of three plants in an uneven triangle, then two more plants to fill in a "tail" works well (visually similar to a Big Dipper formation, but using only five points). Then on the opposite end of strip, for balance, an obtuse triangle of three plants using the same species. This base will tie your strip of plants together, even if you then inter-plant with other species. Using the five-point planting template (or the three-point obtuse triangle), but stretched out farther apart, works for the fill-in plants too (try to group the 3 triangle plants in the wider sections, the two along the narrower sections). There is room for a small (single) specimen plant, or use an artistic dog-rerouter as a focal point; roughly 1/3 of the way up the strip, in the wider section of the bed would be a good location.

My design style for my own home leans towards a mixed garden of shrubs and perennials. If going with salvia, be sure to choose ones that do not have invasive roots. There are some lovely, New Zealand flax plants offered (choose ones that can take hot mid-day sun); they are tough, low maintenance plants that provide wonderful foliage contrast. I understand why the designer recommended them. Kangaroo Paws, tall and short, can flower for months at a stretch. Lavender would be a good choice. Hardy geraniums (watch out for the invasive ones), gailardia, daylilies, alstromeria, coriopsis, angelonia, Kalanchoe cv. mirabilis, are a just few perennials to suggest.

Don't forget bulbs can be a fun addition, too. Plant some freesias, and they will multiply and flower every year without a bit of fuss (CostCo $12 package available in the fall is a great deal for these). Here is a photo of some of mine after three years--I probably need to thin them out! After the freesias are done blooming, my bletilla and russelia continue the show in a slightly more orderly fashion. ;)

You will not like my solution, but with a big dog, a little baby, and more planned, I'd remove everything between the pool deck and the wall, put down artificial turf and call it a day. When the kids hit middle school, by which time your next generation of dog will be settling down, then it will be time to put in the cottage garden of your dreams.

Maybe put something above the wall that drapes over the wall a bit, like a weeping rosemary or Russelia equisetiformis. Seriously, how much time are you going to have to maintain it? Likely even less than now, once baby becomes toddler and is into everything.

Yipee, another young gardener. Who loves cottage gardens as do I. Last night dreams of your garden-to-be ran thru my head. Duh, the entire area can't be planted. OK, realistic plan B.

Firstly get 1-2 shrubs planted to screen off the area beside the house. A green screen there provides a private sandbox area, potting area, hose storage, whatever you like. Diosma is a good shrub. Soft growth and a nice shade of green, good texture, bonus of bloom once a year. It would be look well with the birches and ivy. Nice growth rate.

Let those agapanthus work for you. Take out one more clump. Plant one, or two, Grewia caffra (oops, reclassified as Grewia occidentalis --- Lavender Star Flower) close to the wall between the two agapanthus clumps. I would use another further down as well. No need to attach guide wires to the stucco unless hubby likes doing that kind of thing. It will grow as a natural espalier. Clip off any branches poking outwards when young. Will need a yearly shearing after established. Buy 6-7 (as needed) more Marguerites. Plant them as close as 1 1/2 ft. from the agapanthus (assuming you have marguerites that grow to 3 plus ft.). The agapanthus will then peek out between the marguerite's lacy foliage for excellent contrast and lushness. If you like shasta daisies a couple of clumps would combine well and provide cut flowers. The other plant that would be compatible (I think) are the large flowered hybrid Penstamons. Clumps of solid red or purple gladiolus here and there could be added later. As with all bulbs there is the drying foliage period.

Yes, repeat in a long narrow bed unifies things. Since there is a 3 foot (?) width to work with, your choice of plants is limited. Marguerites are a great choice. I think I would not put more than 3 together (in a row - let your eye guide you). Pieces of it will root if you're on a tight budget and have the time. But no more lacy, ferny plants or it will become messy looking. Buy a few 16-20inch pots (the size which fits the overall scale of things) to place around the pool and indulge your love of annuals in them. Rosemary is great in a pot. And combines nicely with many annuals.

Now the dog has his path and you have a restful, interesting, low maintenance bed to enhance your yard while the children grow. As Hoov said, your dream garden will happen. (how is astroturf on dogs' paws :-) ?

Buy the Sunset Western Garden Book. It was my bible as a young newlywed in a first house. And I still consult it. Take pictures! You'll be glad you did. The 2 hardest things for me to learn were texture combos and scale. I was great on color! Enjoy. irisgal.

PS. plants go in front of parts of the Grewia.

HoovB knows whereof she speaks with the dog issue. But really can you look at that area and not do something. A long narrow area gives you a few choices. You can do the repetitive thing the designer showed you. I have seen very effective repetitive designs with common plants. In San Francisco there was one with a repeat of Shasta Daisies and Agapanthus that was lovely. Repetitive could be alternating plants, alternating colors, alternating texture, alternating height, any balanced combination of the three. -Essentially a simple rhythmic composition.

I would suggest first addressing the hardscape issue. The dog has a path, make it official. Make an undulating path where the dog has made the trail. At least outline the area somehow even if you want to wait to put in gravel, dg, pavers whatever. You need access also.

Put some taller plants at either end. Ceanothus Ray Hartman can be pruned up to walk under, Arctostaphylos Howard McMinn can do the same. For color use Penstemon Margarita BOP, Verbena lilacina De La Mina, Grindelia, Monkey Flowers, Perityle incana etc. For pretty placeholders that take up space until others grow in, Limonium perezii provides flowers, is easy and easy to pull up.
Don't tell HoovB but roses can take alot of neglect and still look good. If you like them there are any number that would work. International Herald Tribune is part Rosa californica with smallish purple flowers.

Use the tall manzanita and ceanothus at both ends. Make sure to give them about 8 feet between them. Plant place holders like the Limonium and Monkey Flowers between them. Then put the others in and maybe leave a 4 foot space add some non plant interest like rocks, interesting driftwood, sculpture, pots, annuals, bulbs etc.

Come to terms with the reality that some will die, some will not work for you and that gardens are all about change. Try stuff you like but don't get attached.

I didn't read the whole thread but how about another retaining wall border about half the size of the existing wall for a raised and layered look...might keep the dog out too with some training. Could serve as a bench along the side of the pool.