Larger shot will take up more space. Opening crimps and re-crimping is a pain. I use the same data for same weight loads from BB's to 9's. I will say that all my loads are as low pressure as will reliably fire in my sxs's. I don't load Buckshot so no help there but know that there is usually different data for buckshot loads.
I agree with this. Loading manuals do not care what size shot is being used, it is the oz. of shot for a given charge weight. The manuals specify hull, powder, charge weight of powder, primer, wad, and oz.'s of shot (not size).
I have not loaded buckshot or slugs so there maybe a difference there, I don't know.