I had the same issue a few weeks ago when I was flying....the TSA website is SO unclear on this stuff!!?! I wanted to know if I could mix up formula BEFORE going through security, and no one could even answer that question for me.
When I got to the airport, I noticed a sign that said "The 3oz liquid rule does NOT apply to milk, baby formula, baby food or medicine". Whew!
The TSA people were really helpful too; whenever they saw that I had a diaper bag, they would automatically tell the scanners to expect baby formula coming through.