It is fine. Insoitol is created from glucose, it is 50% as sweet as sugar so maybe it would even help with the taste of the choline (which smells like fish and probably tastes terrible, I am not gunna try it on its own). Scientists have not tested more then 18g a day of insoitol in people but up to that level it is safe. It may interact with vitamins and minerals such as iron because it can bind with them in the stomache, this may be a concern for a soylent blend. However studies were not conclusive in showing this.
If your kidneys are working fine you will urinate any excess inositol (I believe).
In Short: I would have no problem with using choline+inositol in my own soylent recipe.
I am not a doctor or a nutritionist, however I am an enthusiast and I have read loads of information about nutrition and health. From what I have seen the vitamins that can cause a problem for people in terms of getting too much of them are fat-soluable vitamins such as Vitamin A, D, E, K. Also plant sources of iron are better then supplemental sources because the body can control its absorbtion of the mineral with plant sources while supplement sources flow right into the blood.