I haven't used mini-kegs for over ten years, but just recently picked up a few that I will fill this weekend. In preparation for that I was doing some research and found some documents that will answer your questions:
Long story short - you'll use about a tablespoon per minikeg. Be sure to leave an inch of headspace. You don't have to secondary if you don't want. You'll still get the benefit of 'bulk aging' in a minikeg. It'll take a couple weeks to fully carbonate, but your beer may still be a bit green. Most styles of beer get better with age.
American Stout, Breakfast Stout clone, Rye IPA, Braggot, Roggenbier, Pale Ale, Oat Stout, Apfelwein, Pliny Clone