I believe that Soylent uses Whey protein, because of common allergies to Soy protein... which is amusing, since an intolerance to milk protein runs in my family, and appears to be quite common too.
In fact, an extremely common request of clinical nutritionists is for a non-diary product, especially for ones aimed at children.
I strongly urge Soylent to try and gain the help of someone with experience within the established industry and get their assistance. Whilst us hackers might be happy to take the risk at this stage, someone with industry experience and knowledge of product development and production would bring invaluable skill. Plus, they'd be best placed to answer any questions about FDA issues, etc
Disclaimer: I have a family member who works in this field