For pours - I have been trying 3 different types of products: Clear Tar Gel, GAC 800, and self-leveling gel. Can you give me an idea of when I would use one instead of the others? They all seem to work well with a slight difference in the way they spread.
GAC 800 is best for general pouring around and not wanting to be concerned with size of pour. However, it's best with a little color added as it's a little cloudy. The Clear Tar Gel and SLCG are both good for pours, but you have to be careful about heavy pours creating crazes.

I am experimenting with splashing color on my paintings and I wonder if you have any suggestions on how I can increase the 'stickiness' of the color. So far, I have noticed that to mix the color with your 'Clear Tar Gel' is better than ordinary 'Soft Gel', because the color splashes on to the painting in larger chunks, instead of being scattered in smaller segments. However, I wonder if I can increase this even more by using any of your Acrylic Polymer additives? Or are gels better? Or perhaps you have some other suggestion?
The addition of a medium is a good idea, you may also be able to apply a thin coating to the surface before splashing the colors on. I don't know why you wouldn't have good adhesion to begin with, unless you also use house paints or thin the paints with large amounts of water. I like to use the Fluid Acrylics for splashing and if I were to extend them, I think GAC 500 or Polymer Medium would be my first choices.

Sometimes it is important for me to have a surface that is as leveled as possible. What is the difference between mixing the color with 'Self-Leveling Gel' as opposed to using GAC 500 - Acrylic Polymer for Self-Leveling?
Leveling paint films is all about "rheology". The more of a long, syrupy leveling rheology you can add to a paint, the more it will relax and flow. GAC 500 is the thinnest leveling product, followed by Polymer Medium Gloss, then Self-leveling Clear Gel, and finally Clear Tar Gel. However, Clear Tar Gel is fairly thick, so you may need to add water to the medium to achieve better brushing out. Generally, you can safely add 30% water to GOLDEN products with no major effects on film formation. But, the more water you add, the greater chance of "crazing" (riverbed-like cracking patter in the surface of a pour) you can have.