What companion plants should I use to make a 150 sq ft lavender guild in Northern California?
Which lavenders are you planning to grow? angustifolia? stoechas? other?
And, if you're planning to harvest then consider your ease of access. A wider path is always more comfortable than a sidle track.
Rosa rugosa is very drought tolerant, has attractive flowers and interesting hips.
Myrtle has aromatic foliage and tolerates clipping.
Santolina or Artemisia would also be aromatic as well as silvery-leaved. Spathufolium could be an interesting edging. The silvery one with the purplish cast to the leaves. (Capo Blanco?) They each have yellow flowers but they could be shorn to prevent flowering if so desired.