Use around 70% the amount you'd use of active dry yeast than instant. The instant yeast is "stronger", meaning there's a higher percentage of not-dead yeast cells in an equivalent sample.
I've used mixers both smaller and larger than 20 qts, so I can't say from experience, but it's usually pretty good at mixing from a tenth of maximum capacity to around 85%... also depends on attachment as a dough hook or whisk at low speed probably flings less flour around than a paddle.
"If it's chicken, chicken a la king. If it's fish, fish a la king. If it's turkey, fish a la king." -Bender