Personally, I know nothing about the plant in question, but I did find an article post by one of the professionals here at houzz. (Not a secure site).

http://sageoutdoordesigns.com/plants-that-can-survive-under-california-pepper-schinus-molle-trees/

The allelopathic properties of Schinus molle tend to be overstated. Male plants tend to be more adept at deterring competing weed seedling species but neither gender has a significant impact on more mature plantings. As long as you are not seeding in any new arrivals, you should be fine :-) Just follow the usual recommendations for plants that will tolerate dry shade.

FWIW. most tree species will exhibit some degree of allelopathy but rarely to the extent of limiting or impeding other plantings.

We had a beautiful CA Pepper in our back yard. Nothing would grow under it. However, I think the Salvias were fine about 6 feet away, but I don't know about a shrub with larger roots. Maybe a shrub's roots would be happy growing in the opposite direction if it's a fence not a wall. .

hanging baskets in a pepper tree are beautiful. Also I saw a round seating with 3 containers for planting that goes around tree trunks.