If I'm making my own mixtures and trying to be consistent with the blends, should I be measuring by so many parts by weight, or by volume?
Use the unit of measurement most convenient for your applications and needs. The accuracy of your mixtures, however you decide to create them, is more important than whether you choose weight or volumetric measuring. GOLDEN testing is conducted by weight, as it is very reproducible and because we have the proper equipment. It may not be necessary for most artists to go to such an extreme, but if your goal is to be able to recreate these mixtures with a high degree of accuracy, you should consider measuring by weight.

I am trying to find a piece of equipment suitable for a small studio to be used to put GOLDEN paint colors I mix into standard paint tubes. Do you know of any such device?
It's good to hear you do your own mixing and want to put them into tubes for future use. This is a good idea for artists!

Over the years we have found large plastic syringes work really well for this. You can pull them apart, load them up with a palette knife, and then tube them using the syringe. Also, look for a set of vise grips with a wide area so you can really clamp down as you fold up the crimp.

Suppose I want to paint a layer with a very small amount of color (pigment) relative to the acrylic medium. What is the smallest possible percentage of color for: 1) Clear Gel 2) Self-Leveling Clear Gel 3) Acrylic Glazing Liquid ? Or is it better to use GAC 500 or GAC 700? (Is there any difference if I use Heavy Body Acrylics or Fluid Acrylics?)
There really isn't a minimum or maximum amount of color to add. It's up to the artist to decide.