I can offer advice about Feiyue shoes having worn them for many years.
Although they do have a slightly curved bottom, most people find this flattens quickly with wear so take them out for a good long walk. They do still allow a good range of movement but are more stable than most badminton shoes (which also are designed for movement) because they have a wide fitting. I have never experience any degree of wobble beyond the first week of wearing but then YMMV. Any problem using them for traditional tai chi? Only the same as any rubber soled shoe, they may grip a little too much so you need to pivot carefully.
I would have said they are quite cheap (unless you are looking at the French version of course).
In terms of sizing they go by length so you should be able to find your size easily. We have at sizing chart at http://www.taichifinder.co.uk/feiyue-size-chart
Be warned though that is specific to the traditional style from Top One, Shanghai. Any reference to sizing is to European and UK sizes, not American.