The reason why regular milk is not recommended for children under one year is because the protein is too complex for the baby to digest and can cause damage to their digestive systems and because it can "cause" milk allergies or intolerance later on.
The formula that we use for babies is milk-based, but it is modified to suit their immature digestive systems. I believe yogurt and cheese do not count in this "one year wait" category.
If you want to use soy milk, please research the benefits versus the drawbacks of this substitution so that you can make an informed decision. I have also heard of some babies (including my own) who were soy intolerant. For my nine month old baby I am currently using a combination of small amounts of juice, rice milk, and small amounts of sensitive milk-based formula since she is intolerant of both milk and soy base, and breast milk.
You have to do what works best for you and your baby.