Does this have soy in it?

No it does not. Gluten Free, Dairy Free and Soy Free! Hope to serve you soon! Mom's Place Gluten Free

how much water to add to 1 oz of soup mix

Depends on your usage. I usually would add one cup of water if just eating it as soup. It is also used as a meat tenderizer on roasts, in the crockpot. It is also used to mix in with sour cream as a potato chip dip. It's so versatile just enjoy it! (Just as you would "Lipton" onion soup mix. Except Moms Place version is Gluten Free!)

Can this be used to make cream of onion soup?

You would definitely need to alter it quite a bit, a better solution would be to use our Cream of Onion Soup Mix (it's going to give you a good cream sauce/soup). https://www.amazon.com/dp/B0055J9O5S?ref=myi_title_dp

Does the bouillon in this contain soy?

I can't say for certain whether the bouillon contains soy. You will likely have to contact Mom's Place Gluten Free directly to verify, especially if you are sensitive. 208-529-6667 or momsplaceglutenfree.com