I think the replicator would work nicely for grass. You could fashion a few 'clumps' of grass, enough to break up any sense of monotony, then replicate them in mass to populate a field.
You could try Anything Grows; but I don't think it would be practical for a huge amount of grass.

The volume clouds should be interacting with a scene when you make them. I personally think the shadows look better when the cloud's opacity is set low. If you are using Skylight check the 'add skylight' attribute. And 'participate in global illumination' to have the cloud interact with the scene's GI.
You could try a 'basic shape' and play with the threshold to try to get a single volume cloud to look like many clouds; but I think making individual volumes works better over all. It would depend on your needs.

Friends don't let friends use booleans.