[Lonman]

As we all know running sound for 100's of non-professional singers can be a challange and the normal rules may not apply.
Screamers - Use a good compressor/limiter
Coners - ??Assuming you mean mic cuppers? If you have a sweepable mid on your mixer, do a hard mid cut at approx. 1K & turn the bass/treble up a smidge. If you don't have a sweepable mid, then a hard cut on the midrange, but some boards have their mid settings set at different frequencies & may not always work very well.
Whisperers - Not a whole lot you can do here. I will usually sweep their mid up in the 250-350hz range, cut the high/mid & low mid a bit on the music & back the music volume down a bit.
just plain bad singers - Eq adjustments aren't as critical as compression settings & some effects for the plain bad ones.