For Soylent, glycemic index is not the most important factor. Its glycemic load that is more important. I don’t know if fluid and solid Soylent acts the same way, but i make a raw chocolate Soylent cake(No fructose), and there are plenty of fibers and oats to make it digest and release energy from carbs slower. And i eat it in 6 portions throughout the day. So the energy burst is not that overwhelming for the body.
I won’t recommend fructose in bigger doses. The glycemic index of fructose may be lower than glucose, but that’s just because only the liver can metabolize fructose, and it is not absorbed by your insulin receptors like glucose is, and therefor it is not suitable source of energy. Ethanol is considered to be toxic, in large chronic use. Ethanol is also made from fructose. And they both have almost exactly the same impact on the liver. Though ethanol is metabolized by your brain too(which makes you drunk), while fructose isn’t. Smaller doses of fructose is not THAT bad, but for many other reasons, fructose is not very optimal energy source, and it puts your liver under stress more than glucose, because it requires to be metabolized in complex processes.
So the bottom line is, decrease the glycemic load, not by using fructose, but by adding more fiber and complex carbs. Also consider splitting your soylent into more meals. That is more healthy in the long run, i believe.