Actually I'd probably just fix the eels and the gwhacking, I think Scalis is hard as he needs to be.( Not to mention you could probably cut your difficulty even more by making a change to one of those templates/tactics )
The drop rates on Scalis and Corgul are the same though Scalis doesnt have the guaranteed drops that Corgul has.
Also the drop chance isn't a 1 drop limit. Its a flat chance for the qualifying top attackers.
So 2 attackers = 2 separate chances. 6 attackers is 6 chances.