you can put Rafaam in whatever deck, C'Thun needs a deck with bad minions. How many is enough? You probably want like 20 damage. I'm saying Rafaam generally works better and you can do much more with it and it's still not a good card exactly.
Bad minions? We've seen exactly 2 out of the full 16 cultists. Maybe wait until we've seen all of them before determining whether they are bad or not?