Vee, there is a ton of information already available on mediums. A forum search will turn up all kinds of stuff. Check the threads below. If you have questions about a particular medium, I bet members can provide more focused input.
You don't really have to use a medium in order to glaze. A simple "couch" of linseed or walnut oil, wiped down to a minimum, will work admirably; just paint into it. A teeny smidge of Venice Turpentine or Strasbourg Turpentine mixed into the couching oil will help prevent beading.
As for unwanted gloss ... almost all mediums contain a resin or balsam varnish, so there is going to be some degree of gloss. This all evens out when you apply a final varnish.
Glazing Tutty Fruity
Mediums - Why Do You Use Them?
First adventure in Scumbling/Glazing WIP