I don't know if I am alone in saying so but I actually don't much care for Grinning Demon as its drawback is worse than Juzam/Plague a non-negligible amount of time while its extra power/toughness isn't a faster clock in a vacuum (still 4 turns to kill on its own).
When it comes to efficient casting-cost-to-power ratio, Negator is the best of the best, imo. I would play 4 of them before I even considered Scuta. Hidden Horror is to be avoided outside of say, an Oversold Cemetary theme deck.
I think 4 Negator, 4 Juzam, and 3-4 Plague Sliver is all the creatures you need for any mono black fatty deck. You'd be better off including Hymn/Sinkhole/Duress/Thoughtseize in the slots you were intending to run extra creatures in. Sinkhole is insane in a deck with turn one Negators, btw. There's a reason it used to be called the black Time Walk.
