There's no 2-way PhysX support at the moment, between PhysX particles and Particle Physics operator (used for grains). And you definitely don't want to try and send millions of particles to the PhysX engine.
You can either convert your large chunks into (very) stiff cloth (to the point where they act rigid) and drop them in, and have everything simulated in the same flow with Particle Physics operators, or simulate your PhysX in one flow, enable mesh and then bring it into the next grain flow to act as a collider.
Or a final option is to simulate your grains at a lower res as PhysX (maybe up to 100k particles before it gets too unruly and slow)...and then upres the sand simulation in a separate flow using particle force operators. I haven't produced example files on how to do that yet but it's planned for the future.
You can either convert your large chunks into (very) stiff cloth (to the point where they act rigid) and drop them in, and have everything simulated in the same flow with Particle Physics operators, or simulate your PhysX in one flow, enable mesh and then bring it into the next grain flow to act as a collider.
Or a final option is to simulate your grains at a lower res as PhysX (maybe up to 100k particles before it gets too unruly and slow)...and then upres the sand simulation in a separate flow using particle force operators. I haven't produced example files on how to do that yet but it's planned for the future.