is it possible to add particles to flow simulations and modify their density/size and see how that effects the flow?
Regarding your question - it is possible to simulate flow with particles of different density and size using Simscale (Fluid Dynamics -> Discrete Phase Model). The solver behind this is DPMFoam solver, and it’s description available in the web - DPMFoam includes the effect of the particulate volume fraction on the continuous phase, suitable for dense particle flow simulation. The solver uses existing functionality for particle clouds and their collisions, which directly resolves particle-particle interactions.
You can look up example of this solver application in the Public Project Library and decide if you can use it for your particular case.