Always include the list.
You seem to be asking a few questions, that are not satisfactorily differentiated from one another. Much of what you are asking depends wholly on what you are doing. However, let me touch on two points.
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fix pour knows nothing about your pair style. The way fix pour works is by inserting spheres only where they do not already exist. (This is in the fix documentation)
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Stiffness relates to the dynamics of realistic particles, which are very stiff (try squeezing a rock or ball bearing). What stiffness is good/stiff enough? depends largely on what you are trying to extract and what you’re trying to model. To answer what is good enough, there really is no substitute for reading literature (which is even old by now ->check the pair style doc for a starting place) and performing the simulations when no one’s done the work.
Good luck.