I am not familiar enough with what you can grow in Cali-for-ni-A, so I will only make general recommendations. You have plenty of "verticality" in your landscape and you don'y need anything tall there. Anything tall would eclipse the eventual stunning decumbens anyway. Stick with something low growing, about 18" to 24" tops. Those groundcovers behind look good.
Any clue why the Dioon is croaking?