Grevilleas are very shrubby/billowy so some architectural plants like an Agave or two to add lines and contrast with the shrubbiness would be one suggestion.

Since both G. 'Bonfire' and gaura are wispy, I'd so with a shrub having foliage substance. Unfortunately, one of my favorite 'what heat?' foliage shrubs (Agonis flexuosa 'Nana') is just outside your cold zone (to about 25 degreesF). Unfortunately, my idea book is out of commission for serious binding repair.

I love all the colors that Phormium comes in. Is there a downside to Phormium? Less sun/heat tolerant than agave?

Phormiums' sun/heat tolerance varies depending upon species, or for hybrids, their parentage. P. tenax (rigid, upright foliage on tall plants) is the most rugged. To a lesser degree the hybrids take heat and sun. P. cookianum, which tends to have nice, arching foliage, but will scald without mid-day protection.

Downsides include that many of the hybrids try to revert to green form. I'd recommend checking each you are looking at for this tendency, (look for the word stable; San Marcos Growers' LIST can help with some). The common Sundowner has a tendency to revert, while similar in appearance Maori Chief does not, for example. You can remove reverted foliage, but it is easier if that is infrequent maintenance task.

Several species of mealy bug like to hide in the sheaths in Southern CA and can be very difficult to control without using systemic pesticides. They can be so numerous that they suck the energy from the plant, to the point of death or close to it. I don't know the degree this would be a problem in other CA climates.

I love "little John" -- I'm in Sacramento and mine are full sun, very low water (now that they are established), i have a groundcover type grevillia and they are a great combo. If your grevilia flowers in winter you don't have to worry about the red clashing.

Another great companion with similar watering needs is Bulbine frutescens

Or Yucca instead of phormium, because then I have occasional flowers. Is there a downside to Yucca? Different water needs than Grevillea? Do all Yucca's form a trunk after awhile, are there some that never do?