The minScale value in my case should be 0.3. However, when I set it to -0.3, flipping does still not work as expected (the object rather gets squashed) and the user (using the handles) is able to scale the object to a smaller size than I want him to be able to.
I guess the original poster was right to a certain degree in saying that scaling restraints and flipping are two functions that don't work together well, or am I missing something?