There are many enameling texts available, as well as beginning
enameling classes. Check your local library or Alibris for used
enameling texts and consider enrolling in a class or two.
To answer your initial questions:
The purpose of a trivet is to provide as much structural support as
possible with as little physical contact as possible. Base enamel
and counter enamel are typically not done simultaneously.
As far as a small kiln, they all accommodate some kind of low
trivets. There are a variety of ‘low profile’ trivets available. My
personal choice for teaching is the tabletop Amaco model. It has a
6" diameter chamber, and the depth will accommodate regular trivets.